Fix stdio.h cuserid namespace (bug 19989).

Fix stdio.h cuserid namespace (bug 19989).

stdio.h declares cuserid if __USE_XOPEN.  But this was removed in the
2001 edition of POSIX.

The #endif comment "Use X/Open, but not issue 6." reflects the correct
logic, but does not correspond to the #ifdef.  The use of a correct
libc-hacker.  The online archives for libc-hacker in August 2000 are
broken, but the messages can be found in the qmail archives in
/sourceware1/qmail/lists-sourceware/libc-hacker/archive/26 if you have
shell access to sourceware.

The issue showed up in August 2000 because of a warning about a
non-prototype definition in sysdeps/posix/cuserid.c when there was no
previous prototype declaration.  Since we've now eliminated
non-prototype function definitions, that issue does not apply.  The
other points from that discussion were about whether it should be
included in _GNU_SOURCE; whether _GNU_SOURCE should include
"everything"; whether deprecated interfaces such as this should be
excluded from it; and whether, even given exclusion of deprecated
interfaces, it should apply for deprecations in a version of POSIX
that at that time had not been released.

This patch follows the more conservative approach to a fix of keeping
the interface in _GNU_SOURCE.  That matches how L_cuserid is handled.
I think there is a strong case for eliminating this interface from
_GNU_SOURCE (but this may not automatically be the case for every
interface removed in newer POSIX versions), but then L_cuserid should
also be removed from _GNU_SOURCE (in stdio-common/stdio_lim.h.in) at
the same time.

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

	[BZ #19989]
	* libio/stdio.h (cuserid): Do not declare if
	[__USE_XOPEN2K && !__USE_GNU].
	* conform/Makefile (test-xfail-XOPEN2K8/stdio.h/conform): Remove
	variable.