Make heap growth more conservative after GC_gcollect_and_unmap call

Make heap growth more conservative after GC_gcollect_and_unmap call
(Consider heap size growth since latest GC_gcollect_and_unmap
invocation instead of absolute heap size value when computing amount
of extra memory blocks to add to heap in GC_collect_or_expand.)

* alloc.c (GC_heapsize_at_forced_unmap): New static variable.
* alloc.c (GC_gcollect_and_unmap): Store current heap size to
GC_heapsize_at_forced_unmap; add comment.
* alloc.c (GC_collect_or_expand): Change blocks_to_get computation
considering heap size growth since latest GC_heapsize_at_forced_unmap
call (instead of the absolute heap size value); do not call
GC_expand_hp_inner twice with same argument.
* include/private/gc_priv.h (_GC_arrays._heapsize): Refine comment.