GOLF BAG CART FOLDABLE DEVICE BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a golf bag cart foldable device, more particularly, to one 5 which features a structure that is handy, easy and smooth in the operations. Description of the Prior Art Prior golf bag cart foldable devices, approximately like the one shown in FIG. 1, have an interlocking mechanism 10 set up between its upper cart stand 11 and bottom cart stand 12. When demanding a fold, the buckle 13 of the interlocking 10 mechanism 10 is first pulled up, and the upper cart stand 11 is pushed forward and down, to have rear wheel frame 16 bent for a fold by means of the connection by a linkage 14 and a strut bar 15. Reversely, once demanding a unfold, the upper cart stand 11 is pulled up, to unfold the rear wheel frame 16 by means of the connection by the linkage 14 and the strut bar 15, and the buckle 13 has to be 15 pushed down when the rear wheel frame 16 is bent to its position, to interlock the upper cart stand 11 and the bottom cart stand 12, followed by a forward release of its front wheel 17 to come to the unfolded position. Due to the location of the buckle 13 of the interlocking mechanism 10 is arranged at the bottom portion of the upper cart stand 11, which is low in position that the 20 user has to bend down to pull the buckle 13 for the fixing of the upper cart stand 11 and the bottom cart stand 12 once demanding a fold. And it means frequent uses surely fatigue the user which apparently deserves a great improvement. SUMMARY OF THE INVENTION The objective of this invention is to provide a golf bag cart foldable device, which features handiness, easiness and smoothness in the operation that is capable of improving the drawbacks of the prior art. To achieve the aforementioned objective, this invention comprises: a bottom cart 5 stand, having a link segment at its top, where the inner wall of the link segment is set up with an indent, and the internal side of the bottom cart stand is provided with a channel, offering for the sliding by a slider, where the far end of the bottom cart stand joins a front wheel; an upper cart stand, in pinned connection with the link segment at its bottom end, and having a long trough at the inner side of its 10 bottom end; a rear wheel frame, in pinned connection with the link segment at its one end, and to a rear wheel at the other end; a linkage, connected in between the slider and the upper cart stand; a strut bar, connected in between the slider and the rear wheel frame; and an interlocking mechanism, where the interlocking mechanism comprises: a pull handle, set up in the middle of the top portion of the 15 upper cart stand; a clamping rod, set up in the middle of the bottom portion of the upper cart stand, and each of its two ends is lodged in a long trough; a connection member, hidden in the pull handle and in the upper cart stand, where its two ends are connected to the two ends of the clamping rod respectively; and a spring, joined to the clamping rod at its one end, and to the upper cart stand at the other 20 end. Accordingly, by means of the aforementioned structure, the operations of unfolding and folding would feature handiness, easiness and smoothness. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a three-dimensional diagram of a prior golf bag cart; 2 FIG. 2 is a three-dimensional diagram according to the present invention; FIG. 3 is a side view according to the present invention; FIG. 4 is an elevational view of the interlocking mechanism according to the present invention; 5 FIG. 5 is an outer side sectional view of the interlocking mechanism according to the present invention; FIG. 6 is an inner side sectional view of the interlocking mechanism according to the present invention; FIG. 7 is a schematic diagram of the motion of the interlocking mechanism 10 according to the present invention; FIG. 8 is a schematic side view during the folding according to the present invention; and FIG. 9 is a schematic side view of the completed folding according to the present invention. 15 DETAILED DESCRIPTION OF THE INVENTION The structural traits and the functions of this invention are detailed described with reference to the following preferred embodiment and the accompanying drawings, which would help the honorable Examiner to comprehend thoroughly the present invention. 20 Referring to FIGS. 2 - 6, the golf bag cart of this invention as shown has a structure of bilateral symmetry, and the following description is unilateral for the sake of convenience. This invention comprises: a bottom cart stand 2, having a link segment 21 at its top, where the link segment 3 21 takes on a U-shaped cross-section and forms a ramp 211 at its front portion, and is provided with an indent 212 on top of its inner wall, the bottom cart stand 2 is provided with a bag base 22 and a front wheel 23 at its bottom, and has a channel 24 along its internal side, for accommodating the sliding by a slider 25; 5 an upper cart stand 3, in pinned connection with the link segment 21 at its bottom end, being provided with an axle 31 in its internal, and having a long trough 32 at its internal side; a rear wheel frame 4, in pinned connection with the link segment 21 at its one end, and to a rear wheel 40 at the other end; 10 a linkage 41, connected in between the slider 25 and the upper cart stand 3, to generate the connection between the upper cart stand 3 and the slider 25; a strut bar 42, connected in between the slider 25 and the rear wheel frame 4 to generate the connection between the slider 25 and the rear wheel frame 4; and an interlocking mechanism 5, comprising: a pull handle 51, set up in the middle of 15 the top portion of the upper cart stand 3; a clamping rod 52, set up in the middle of the bottom portion of the upper cart stand 3, and each of its two ends is lodged in a long trough 32; a connection member 53, for instance: a steel rope, hidden in the pull handle 51 and in the upper cart stand 3, where its two ends are connected to the two ends of the clamping rod 52 respectively; and a spring 54, joined to the 20 clamping rod 52 at its one end, and to the axle 31 of the upper cart stand 3 at the other end, which is able to pull down the clamping rod 52 for being clamped by the indent 212 of the link segment 21. According to the aforesaid structural combination, the unfolding of this invention 4 is shown in FIGS. 2 & 3, where the spring 54 of the interlocking mechanism 5 is in a normal state, and the clamping rod 52 is clamped in the indent 212, shown in FIGS. 4 - 6. For desiring a fold, only pull the pull handle 51 up, shown by the arrow in the FIG. 4, and through the driving by the connecting member 53, the 5 clamping rod 52 moves up along the long trough 32 and releases from the indent 212 of the link segment 21, followed by pushing the upper cart stand 3 forward and down to release the pull handle 51, shown in FIGS. 7 & 8, which the clamping rod 52 is to slide down along the ramps 211 at both sides of the link segment 21, and through the support of the resilience of the spring 54, the upper 10 cart stand 3 is ready to be folded down. Meanwhile, the slider 25 driven by the linkage 41 moves forward on the bottom cart stand 2, and simultaneously pulls the rear wheel frame 4 to move forward and inward through the pulling by the strut bar 42, which further drives the rear wheels 40 to move forward and inward for a folding, and finally the front wheel 23 is turned downward and inward for a fold 15 at the underside of the bottom cart stand 2, shown in FIG. 9, to accomplish the folding process. Reversely, for desiring an unfold, the front wheel 23 is first turned upward and outward to its position, followed by pushing the upper cart stand up, and by means of the connection by the linkage 41, the slider 25, the strut bar 42 and the rear 20 wheel frame 4, the slider 25 is forced to slide toward the link segment 21, which enables the rear wheels to unfold to its position. At the moment the upper cart stand 3 is pulled up, the clamping rod 52 slides up along the ramps 211, forcing the spring 54 to stretch, until the clamping rod 52 moves to the top of the indent 5 212, then by means of the resilience of the spring 54, the clamping rod 52 falls right into the indent 212, which enables the upper cart stand 3 to unfold to its position, and enables the upper cart stand 3 and the bottom cart stand 2 to interlock each other, to accomplish the unfolding process, and which is shown in 5 FIGS. 2, 3 & 6. From the aforesaid description, this invention features at least the following advantages and functions, which is much creative than the prior art: 1. During the action of folding, since the pull handle 51 of the interlocking mechanism 5 is set up at the top portion of the upper cart stand 3, which is 10 high enough in position that the user is handy to operate without the need for bending one's back, and it substantially reduces the fatigues during the operation. And it is indeed handy, easy and smooth in the operations. 2. During the action of unfolding, the upper cart stand 3 and the bottom cart stand 2 are pushed to a specific widened position, where the clamping rod 15 52 of the interlocking mechanism 5 and the indent 212 of the link segment 21 are to be interlocked each other automatically, at the moment there is no need for an additional step to lock the interlocking mechanism 5, which enables the upper cart stand 3 and the bottom cart stand 2 to be placed to their positions. And this is surely easier in the operations. 20 To sum up, the disclosed structure of the preferred embodiment of this invention is not only unknown to the prior art, but surely can accomplish the expected objectives and functions, which is construed as novel and creative, and a utility patent application is then filed according to the patent law. 6