Make sys/stat.h define S_IFSOCK, S_ISSOCK for XPG4 (bug 20076).

Make sys/stat.h define S_IFSOCK, S_ISSOCK for XPG4 (bug 20076).

sys/stat.h should define S_IFSOCK and S_ISSOCK for XPG4 (XNS), but
does not.  This patch corrects the relevant header conditionals.

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

	[BZ #20076]
	* io/sys/stat.h (S_IFSOCK): Define for [__USE_XOPEN_EXTENDED]
	instead of [__USE_UNIX98].
	(S_ISSOCK): Likewise.
	* conform/Makefile (test-xfail-XPG4/sys/stat.h/conform): Remove
	variable.