conformtest: Correct signal.h expectations for XPG4 / XPG42.

Various of the signal.h conform/ failures for XPG4 and XPG42 are
actually the result of incorrect test expectations rather than header
bugs.  This patch fixes the expectations to accord with those
standards (this does not however allow any XFAILs to be removed, as
some header bugs remain).  Note for anyone comparing with the
standards: corrigendum U013/16 removes the mention of a sigmask
function in signal.h (C435 lists such a function in the definition of
signal.h, but without any actual specification for the function
itself), so sigmask is not included in the expectations.

Tested for x86_64.

	* conform/data/signal.h-data (sa_sigaction): Do not expect for
	[XPG4].
	(SA_SIGINFO): Likewise.
	(SA_ONSTACK): Likewise.
	(SA_RESETHAND): Likewise.
	(SA_RESTART): Likewise.
	(SA_NOCLDWAIT): Likewise.
	(SA_NODEFER): Likewise.
	(SS_ONSTACK): Likewise.
	(SS_DISABLE): Likewise.
	(MINSIGSTKSZ): Likewise.
	(SIGSTKSZ): Likewise.
	(ucontext_t): Likewise.
	(stack_t): Likewise.
	(struct sigstack): Likewise.
	(SI_USER): Do not expect for [XPG4 || XPG42].
	(SI_QUEUE): Likewise.
	(SI_TIMER): Likewise.
	(SI_ASYNCIO): Likewise.
	(SI_MESGQ): Likewise.
	(bsd_signal): Do not expect for [XPG4].
	(killpg): Likewise.
	(sigaltstack): Likewise.
	(sighold): Likewise.
	(sigignore): Likewise.
	(siginterrupt): Likewise.
	(sigpause): Likewise.
	(sigrelse): Likewise.
	(sigset): Likewise.
	(sigwait): Do not expect for [XPG4 || XPG42].