conformtest: clean up POSIX expectations for stdlib.h, string.h.

conformtest: clean up POSIX expectations for stdlib.h, string.h.

Continuing the series of patches to clean up conformtest expectations
for "POSIX" (1995/6) based on review of the expectations against the
standard, this patch cleans up expectations for stdlib.h and
string.h.  Tested x86_64; no new XFAILs needed.

	* conform/data/stdlib.h-data [POSIX] (stddef.h): Do not allow
	header inclusion.
	[POSIX] (limits.h): Likewise.
	[POSIX] (math.h): Likewise.
	[POSIX] (sys/wait.h): Likewise.
	* conform/data/string.h-data [POSIX || UNIX98] (strtok_r): Require
	function.
	[POSIX] (stddef.h): Do not allow header inclusion.