Fix psignal, psiginfo declaration conditions (bug 18483).

Fix psignal, psiginfo declaration conditions (bug 18483).

signal.h declares psignal and psiginfo if __USE_XOPEN2K - that is, for
the 2001 edition of POSIX.  These functions were actually added in the
2008 edition (as indicated in the header comments).  This patch fixes
the header conditionals.  This fixes some linknamespace test failures
because psiginfo uses fmemopen, which is also new in the 2008 edition,
so before the header fix this appeared to the linknamespace tests as a
2001 function bringing in references to a 2008 function.  The problem
also appeared in conformtest header namespace test results (the
conformtest data has correct conditionals for when these functions
should be visible), but the affected headers still have other
namespace problems so this doesn't fix any of those XFAILs.

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).

	[BZ #18483]
	* signal/signal.h [__USE_XOPEN2K] (psignal): Change condition to
	[__USE_XOPEN2K8].  Remove redundant #endif.
	[__USE_XOPEN2K] (psiginfo): Change condition to [__USE_XOPEN2K8].
	Remove redundant #if.
	* conform/Makefile (test-xfail-XOPEN2K/signal.h/linknamespace):
	Remove variable.
	(test-xfail-XOPEN2K/sys/wait.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K/ucontext.h/linknamespace): Likewise.