obstack: use int where glibc does

obstack: use int where glibc does

Make it easier to merge with glibc, by using int where
the glibc API uses int, when compiling for glibc.
This does not affect Gnulib behavior.
* lib/obstack.in.h (_OBSTACK_INDEX_T): New macro.
* lib/obstack.in.h (struct obstack):
* lib/obstack.c (_obstack_begin_worker, obstack_begin)
(_obstack_begin_1, _obstack_newchunk, _obstack_memory_used):
Use _OBSTACK_INDEX_T, not _OBSTACK_SIZE_T, for API components
where glibc uses int.