Fix stdio.h namespace for pre-threads POSIX (bug 20014).

stdio.h declares flockfile, ftrylockfile, funlockfile, getc_unlocked,
getchar_unlocked, putc_unlocked and putchar_unlocked if __USE_POSIX,
with comments "These are defined in POSIX.1:1996.".  But __USE_POSIX
is actually POSIX.1:1990, and these functions should not be declared
for 1990 / 1992 / 1993 POSIX, XPG3 or XPG4.  This patch fixes stdio.h
to use __USE_POSIX199506 instead for those conditionals, as that is
the correct conditional for the version of POSIX that introduced
threads, and with threads those functions.

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

	[BZ #20014]
	* libio/stdio.h (getc_unlocked): Declare if [__USE_POSIX199506],
	not [__USE_POSIX].
	(getchar_unlocked): Likewise.
	(putc_unlocked): Likewise.
	(putchar_unlocked): Likewise.
	(flockfile): Likewise.
	(ftrylockfile): Likewise.
	(funlockfile): Likewise.
	* conform/Makefile (test-xfail-XPG3/stdio.h/conform): Remove
	variable.
	(test-xfail-XPG4/stdio.h/conform): Likewise.