Fix netdb.h addrinfo namespace (bug 18529).

netdb.h declares interfaces such as getaddrinfo if __USE_POSIX,
i.e. POSIX.1:1990 or later.  However, these interfaces were new in the
2001 edition of POSIX, although the header was in XPG4 and UNIX98, so
they should not be declared for XPG4 or UNIX98.  (This produces
spurious linknamespace test failures, although there are other
failures for this header as well for the same standards so this patch
doesn't remove any XFAILs.)  This patch corrects the condition, and
the conform/ test expectations which were similarly wrong.

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

	[BZ #18529]
	* resolv/netdb.h [__USE_POSIX]: Change condition to
	[__USE_XOPEN2K].
	* conform/data/netdb.h-data [XPG4 || UNIX98] (struct addrinfo): Do
	not expect.
	[XPG4 || UNIX98] (AI_PASSIVE): Likewise.
	[XPG4 || UNIX98] (AI_CANONNAME): Likewise.
	[XPG4 || UNIX98] (AI_NUMERICHOST): Likewise.
	[XPG4 || UNIX98] (AI_V4MAPPED): Likewise.
	[XPG4 || UNIX98] (AI_ALL): Likewise.
	[XPG4 || UNIX98] (AI_ADDRCONFIG): Likewise.
	[XPG4 || UNIX98] (AI_NUMERICSERV): Likewise.
	[XPG4 || UNIX98] (NI_NOFQDN): Likewise.
	[XPG4 || UNIX98] (NI_NUMERICHOST): Likewise.
	[XPG4 || UNIX98] (NI_NAMEREQD): Likewise.
	[XPG4 || UNIX98] (NI_NUMERICSERV): Likewise.
	[XPG4 || UNIX98] (NI_DGRAM): Likewise.
	[XPG4 || UNIX98] (EAI_AGAIN): Likewise.
	[XPG4 || UNIX98] (EAI_BADFLAGS): Likewise.
	[XPG4 || UNIX98] (EAI_FAIL): Likewise.
	[XPG4 || UNIX98] (EAI_FAMILY): Likewise.
	[XPG4 || UNIX98] (EAI_MEMORY): Likewise.
	[XPG4 || UNIX98] (EAI_NONAME): Likewise.
	[XPG4 || UNIX98] (EAI_SERVICE): Likewise.
	[XPG4 || UNIX98] (EAI_SOCKTYPE): Likewise.
	[XPG4 || UNIX98] (EAI_SYSTEM): Likewise.
	[XPG4 || UNIX98] (EAI_SYSTEM): Likewise.
	[XPG4 || UNIX98] (freeaddrinfo): Likewise.
	[XPG4 || UNIX98] (gai_strerror): Likewise.
	[XPG4 || UNIX98] (getaddrinfo): Likewise.
	[XPG4 || UNIX98] (getnameinfo): Likewise.