conformtest: Add langinfo.h expectations for YESSTR, NOSTR.

The conformtest expectations for langinfo.h fail to include the YESSTR
and NOSTR constants that were present in UNIX98 and earlier XPG
standards.  This patch adds those expectations, so fixing three
XFAILs.

Tested for x86_64 and x86.

	* conform/data/langinfo.h-data [XPG3 || XPG4 || UNIX98] (YESSTR):
	Expect constant.
	[XPG3 || XPG4 || UNIX98] (NOSTR): Likewise.
	* conform/Makefile (test-xfail-XPG3/langinfo.h/conform): Remove
	variable.
	(test-xfail-XPG4/langinfo.h/conform): Likewise.
	(test-xfail-UNIX98/langinfo.h/conform): Likewise.