obstack: size_t → _OBSTACK_CHUNK_SIZE_T

obstack: size_t → _OBSTACK_CHUNK_SIZE_T

* lib/obstack.c (_obstack_begin, _obstack_begin_1):
* lib/obstack.in.h (struct obstack, obstack_init, obstack_begin)
(obstack_specify_allocation)
(obstack_specify_allocation_with_arg, obstack_chunkfun):
Use _OBSTACK_CHUNK_SIZE_T instead of size_t in places where
glibc uses unsigned long rather than size_t.  This makes
no difference with Gnulib, but should help allay any concerns
that merging Gnulib with glibc might change glibc’s ABI
on obsolescent glibc platforms where size_t is unsigned int.