FOLDABLE GOLF BAG CART BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a foldable golf bag cart and, more particularly, to one that 5 features simultaneous unfolding and folding for an upper cart stand, a bottom cart stand, a front wheel and rear wheels, to facilitate the handiness in use Description of the Prior Art Prior foldable golf bag carts are of a general shape as shown in FIG 11, where an upper cart stand 7 (that is, the handle bar) and a rear wheel frame 71 are joined 10 together by a strut bar 72. Folding of the front wheel 74 and of the rear wheels 73 are independent of each other, i.e they are unable to be folded simultaneously. That means that a user has to squat down to operate a front wheel folding joint 75 and then pull the front wheel 74 for a frontward (or backward) folding, for the completion of the folding of the golf bag cart 15 On the other hand, the unfolding of the cart for use also demands that a user has to squat down to operate the front wheel 74, and this is inconvenient, especially in those situations in which the front wheel 74 and the rear wheels 73 are covered with dirt, weeds or/and mud if the cart has already been moved over the golf course. And the folding of the front wheel 74 at this moment would no doubt smudge the hands and 20 the clothes of the user. It is an object of the present invention to provide an improved foldable golf bag cart SUMMARY OF THE INVENTION According to the present invention there is provided a foldable golf bag cart comprising: 1 ) a bottom cart stand, having a joint at its top, where the joint is in pinned 5 connection with a rear wheel frame at each of its two sides to connect to rear wheels, and is connected to a bag base at its bottom, and the bottom of the bag base isjoined to a front wheel frame to connect to a front wheel; 2..) an upper cart stand, having a winding member at its bottom to pivotally connect to the joint; 1 0 3..) a sliding adjustment structure, set up on the bottom cart stand and capable of sliding along the bottom cart stand, being made up of a primary sliding formation and a secondary sliding formation; 4 ) a link set, made up of a plurality of links, connecting to the upper cart stand, the bottom cart stand, the front wheel frame, the rear wheel frames and the sliding 15 adjustment structure respectively, to form links between the upper cart stand and the bottom cart stand and between the front wheel frame and the rear wheel frame; and 5.) a joint located on the winding member of the upper cart stand and having an operating handle, and the upper cart stand being moved, by means of the link set 20 in such marker that unfolding or folding of the upper cart stand, the bottom cart stand, the front wheel and the rear wheels can be accomplished BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a perspective view of a foldable golf bag cart according to the present invention; 2 FIG 2 is a side view of the foldable golf bag cart according to the present invention; FIG 3 is an exploded perspective view of the foldable golf bag cart according to the present invention; FIGS 4 and 5 are sectional views of the golf bag cart according to the present 5 invention; FIG 6 is an exploded view of part of the golf bag cart according to the present invention; FIG 7 is a sectional view of the part of the golf bag cart shown in Figure 6; FIG 8 is a finther sectional view illustrating unlocking of a joint of the golf bag cart 10 according to the present invention; FIG 9 is a schematic side view illustrating folding of the golf bag cart according to the present invention; FIG 10 is a schematic side view illustrating the completely folded golf bag cart according to the present invention; 15 FIG 11 is a perspective view illustrating the prior art golf bag cart referred to above; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1-7, the foldable golf bag cart comprises: a) a bottom cart stand 1, having a plate-like joint element 11 at its top, where the joint element 11 is in pinned connection with a rear wheel frame 12 at each of its two sides, 20 and the outer end of the rear wheel frame 12 carries a pair of rear wheels 13 There is a fixing formation 14 at the bottom of the stand 1 that is pivotally connected to a bag base 15, and the bag base 15 is further joined fixedly to a front wheel frame 16.. The outer end of the front wheel frame 16 is connected to a front wheel 17, and the bottom cart stand 1 is formed with a longitudinal groove 18 in each of its two sides.. The 3 fixing formation 14 and the bag base 15 each have a positioning face 141, ! 51; b) a sliding adjustment structure, set up on the bottom cart stand I and capable of sliding along the bottom cart stand I, being made up of a primary sliding formation 2 and a secondary sliding formation 3 The primary sliding formation 2 is set up at the 5 top of the bottom cart stand 1 and closer to the joint 11, has an elongated track 21 at each of its two inner sides, which correspond to the longitudinal grooves 18 of the bottom cart stand 1 respectively, and is connected to a third link 53 at its bottom by means of a fixing element 22 (for instance: a bolt). The secondary sliding formation 3 is located at the bottom of the bottom cart stand 1, has an elongated track 31 at each of 10 its two inner sides which correspond to the track grooves 18, and is connected to the third link 53 and to a fourth link 54 at its bottom by means of a fixing element 32; c) an upper cart stand 4, having a winding member 41 at its bottom which is pivotally connected to the joint 11 of the bottom cart stand I, being pivotally connected to an upper mount 42 at its top; 15 d) a link set 5, made up of a plurality of links, is connected to the upper cart stand 4, the bottom cart stand I, the front wheel framel6, the rear wheel frames 12 and the sliding adjustment structure, to form links between the upper cart stand 4 with the bottom cart stand I and the front wheel frame 16 with the rear wheel frames 12.. The link set 5 comprises at least a first link 51, a second link 52, a third link 53 and a 20 fourth link 54. The first link 51 is coupled to the winding member 41 at its top and to the primary sliding formation 2 at its bottom, to form a link between the upper cart stand 4 and the bottom cart stand 1. The second link 52 is coupled to the primary sliding formation 2 at its one end and to the rear wheel frames 12 at its other end, to form links between the upper cart stand 4 and the rear wheel frames 12. The third link 4 53 is located at the bottom of the bottom cart stand 1 and is coupled to the primary sliding formation 2 and the secondary sliding formation 3, where its front part (located adjacent the primary sliding formation 2) is formed with an elongated indent 531 that receives the fixing element 22, to form an indent mechanism, which enables the 5 primary sliding formation 2 not only to have a path constraint during its sliding but to drive the secondary sliding formation 3 to move relative to the third link 53. The fourth link 54 is coupled to the secondary sliding formation 3 at its one end and to the bag base 15 (and thus to the front wheel frame 16) at its other end, to form a link between the upper cart stand 4 and the fiont wheel 17 (via the fiont wheel frame 16) 1 0 through the links between the first link 51, the primary sliding formation 2, the third link 53 and the secondary sliding formation 3 and the fourth link 54; and e) a folding joint 6, set up on the winding member 41 of the upper cart stand 4, comprising: an operating handle 61, in pinned connection with one side of the winding member 41; two movable gear wheels 62, located in the inner side of the winding 15 member 41; two fixed gear wheels 63, located on the top outer side of the joint 11, to correspond to the movable gear wheels 62 respectively; and two springs 64, where each is positioned between a fixed gear wheel 63 and the corresponding movable gear wheel 62, which generates a force to press the movable gear wheel 62 outwards; once the operating handle 61 is pressed upwards for a tightening operation, the inward 20 moving of the movable gear wheel 62 is used to engage the corresponding fixed gear wheel63, so that by fixing the winding member 41, the angle formed by the upper cart stand 4 and the bottom cart stand 1 can be locked; on the other hand, once the operating handle 61 is pressed downwards for a releasing operation, the movable gear wheel 62 is pushed outwards to escape from the corresponding fixed gear wheel 63, 5 subject to the resilient force of the spring 64, which enables the winding member 41 to move and spin With the aforementioned structural combination, the foldable golf bag cart of this invention is in the unfolding state under normal conditions, as shown in FIGS.. 1 & 2, 5 the operating handle 61 at the moment is in its upward position (as shown in FIG 7), which enables the relative position of the upper cart stand 4 and the bottom cart stand 1 to be fixed.. And the rear wheel frames 12 are fixed in the unfolding position of the bag cart, subject to the mutual reaction of the first link 51 and the second link 52, while the front wheel frame 16 is unable to move upwards to be folded, subject to the I 0 mutual reaction of the third link 53 and the fourth link 54. The bag base 15 and the fixing formation 14 of the bottom cart stand I each have a positioning face 141, or 151 located between them and both fit against each other, as shown in FIG 3.. This limits the downward angle that the bag cart 15 is relative to the bottom cart stand 1; therefore, once the bag cart 15 is fixed in the unfolding state, there is no need to 15 immobilize any other joints. If a folding operation is required, the operating handle 61 is loosened first (pull the operating handle 61 down) to release the folding joint 6, shown in FIG 8, followed by pulling the upper cart stand 4 down, and the first link 51 is driven by the winding member 41, which drives the primary sliding formation 2 to move forward (down) 20 along the bottom cart stand 1, shown in FIG 9. Meanwhile, the primary sliding formation 2 pulls the rear wheel frame 12.tlrough the second link 52, to have it move forward and inward, and the rear wheels 13 follow suit to move forward and inward As the primary sliding formation 2 slides to the end of the elongated indent 531 of the third link 53, it starts to press the third link 53 and to drive the secondary sliding 6 formation 3 to move forward (down). At the same time, the fourth link 54 is linked to drive the bag cart 15 and the front wheel frame 16 to move up for being folded, and the front wheel 17 is folded upward too. And the operating handle 61 is then pulled up to lock the folding joint 6, which enables the upper cart stand 4 and the bottom cart 5 stand I to be fixed, to accomplish the folding of the entire bag cart, as shown in FIG 10 Briefly, just by moving the operating handle 61 and pulling the upper cart stand 4, the simultaneous folding for the upper cart stand 4, the bottom cart stand 1, the front wheel 17 and the rear wheels 13 can be accomplished, which substantially facilitate 10 the handiness in use. On the other hand, once an unfolding is desired, the operating handle 61 has to be moved first, followed by an upward lifting of the upper cart stand 4, and the simultaneous unfolding of the front wheel 17, the rear wheels 13, the upper cart stand 4 and the bottom cart stand 1 can then be accomplished, subject to the links between 15 the first link 51, the second link 52, the third link 53 and the fourth link 54 and the primary sliding formation 2, the secondary sliding formation 3 with the front wheel frame 16 and the rear wheel frame 12. Once the unfolding is in place, the operating handle 61 is locked to have the upper cart stand 4 and the bottom cart stand 1 secured to each other, which completes the unfolding of the entire bag cart, shown in FIGS I 20 & 2, considerably to facilitate the handiness in use. From the aforesaid description, this invention features at least the following advantages and functions, which is much more creative than the prior art: I The upper cart stand 4, the bottom cart stand I, the front wheel 17 and the rear wheels 13 can be unfolded or folded simultaneously, which is truly handy in 7 use 2 On folding of the cart, the mutual locking of the upper cart stand 4 and the bottom cart stand 1 provides for handy carrying and storing 3 During the unfolding, the folding joint 6 is responsible for anchoring the cart 4 and there is no need for additional joints to be fixed, which is truly handy in use. 8