xcalloc depending on htab->return_allocation_failure. Return zero
if calloc fails.
(htab_create): Update comment to cover memory allocation.
(htab_try_create): New.
(htab_find_slot_with_hash): Return NULL if htab_expand fails.
Update comment to cover this.
if HAVE_STRING_H.
* pexecute.c, xexit.c: Include stdlib.h if HAVE_STDLIB_H.
* objalloc.c: Include config.h. Include stdlib.h and don't
declare malloc or free if HAVE_STDLIB_H.
* strerror.c, strsignal.c: Include stdlib.h if HAVE_STDLIB_H,
else declare malloc without prototype. Include string.h if
HAVE_STRING_H, else declare memset without prototype. Don't
include stddef.h.
* sort.c: Bring over from GCC.
