hashtab.c, [...]: Include string.h if HAVE_STRING_H.

hashtab.c, [...]: Include string.h if HAVE_STRING_H.

	* hashtab.c, partition.c, sort.c, xmemdup.c: Include string.h
	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.

From-SVN: r34254