Fix waitid namespace (bug 21561).

Fix waitid namespace (bug 21561).

In sys/wait.h, waitid and associated constants and types are UX-shaded
in XPG4.2 (so not in XPG4), and XSI-shaded in POSIX before 2008, so
should be appropriately conditional in the headers.  This patch fixes
the conditionals accordingly.  (WCONTINUED is actually still
XSI-shaded in POSIX.1:2008, but W* is also reserved there without
XSI-shading, so nothing special needs to be done about the
conditionals on WCONTINUED to conform to POSIX.1:2008 namespace
rules.)

Tested for x86_64.

	[BZ #21561]
	* posix/sys/wait.h (idtype_t): Change [__USE_XOPEN] condition to
	[__USE_XOPEN_EXTENDED].
	(id_t): Likewise.
	(include of <bits/types/siginfo_t.h): Likewise.
	(waitid): Likewise.
	* sysdeps/unix/sysv/linux/bits/waitflags.h (WSTOPPED): Condition
	on [__USE_XOPEN_EXTENDED || __USE_XOPEN2K8].
	(WEXITED): Likewise.
	(WCONTINUED): Likewise.
	(WNOWAIT): Likewise.
	* conform/Makefile (test-xfail-XPG4/stdlib.h/conform): Remove.
	(test-xfail-XPG4/sys/wait.h/conform): Likewise.
	(test-xfail-POSIX/sys/wait.h/conform): Likewise.