conformtest: Correct some signal.h expectations for XOPEN2K.

conformtest: Correct some signal.h expectations for XOPEN2K.

The conformtest expectations for signal.h have various declarations
that are expected for POSIX (1996) and all later standards, except,
wrongly, for XOPEN2K.  This shows up as failures of tests for two
other headers, which are allowed to make visible symbols from
signal.h, because of an incorrect namespace failure for sigval
(required in signal.h in XOPEN2K, so should be allowed for those other
headers); signal.h tests for various standards fail anyway because of
other problems in the header.  This patch fixes the incorrect
expectations and removes the two XFAILs that this fixes.

Tested for x86_64 and x86.

	* conform/data/signal.h-data (union sigval): Expect also if
	[XOPEN2K].
	(struct sigevent): Likewise.
	(SIGEV_NONE): Likewise.
	(SIGEV_SIGNAL): Likewise.
	(SIGEV_THREAD): Likewise.
	(SIGRTMIN): Likewise.
	(SIGRTMAX): Likewise.
	* conform/Makefile (test-xfail-XOPEN2K/aio.h/conform): Remove
	variable.
	(test-xfail-XOPEN2K/mqueue.h/conform): Likewise.