(syms_of_alloc) <memory-full>: Doc fix.

(syms_of_alloc) <memory-full>: Doc fix.
(Fmemory_full_p): Function deleted.
(syms_of_alloc): Don't defsubr it.
(memory_full_cons_threshold): New variable.
(spare_memory): Now a vector of 7 elts.
(buffer_memory_full): Don't set Vmemory_full here.
(xfree): Don't try to refill here.
(emacs_blocked_free): Record BYTES_USED in local var.
(memory_full): Now free all the slots in spare_memory.
(refill_memory_reserve): Allocate each slot in spare_memory.
(init_alloc_once): Call refill_memory_reserve.