Fix resolver if_* namespace (bug 17717).

Fix resolver if_* namespace (bug 17717).

Resolver code, brought in by pthreads (at least), uses if_* interfaces
that weren't in POSIX before 2001, resulting in linknamespace
failures.  This patch changes those interfaces to be weak aliases of
__if_* and makes the resolver use __if_* directly.

Tested for x86_64 (testsuite, and that disassembly of installed shared
libraries is unchanged by this patch).

	[BZ #17717]
	* inet/if_index.c (if_nametoindex): Rename to __if_nametoindex and
	define as weak alias of __if_nametoindex.  Use libc_hidden_weak.
	(if_indextoname): Rename to __if_indextoname and define as weak
	alias of __if_indextoname.  Use libc_hidden_weak.
	(if_freenameindex): Rename to __if_freenameindex and define as
	weak alias of __if_freenameindex.
	(if_nameindex): Rename to __if_nameindex and define as weak alias
	of __if_nameindex.
	* sysdeps/mach/hurd/if_index.c (if_nametoindex): Rename to
	__if_nametoindex and define as weak alias of __if_nametoindex.
	Use libc_hidden_weak.
	(if_freenameindex): Rename to __if_freenameindex and define as
	weak alias of __if_freenameindex.
	(if_nameindex): Rename to __if_nameindex and define as weak alias
	of __if_nameindex.
	(if_indextoname): Rename to __if_indextoname and define as weak
	alias of __if_indextoname.  Use libc_hidden_weak.
	* sysdeps/unix/sysv/linux/if_index.c (if_nametoindex): Rename to
	__if_nametoindex and define as weak alias of __if_nametoindex.
	Use libc_hidden_weak.
	(if_freenameindex): Rename to __if_freenameindex and define as
	weak alias of __if_freenameindex.  Use libc_hidden_weak.
	(if_nameindex_netlink): Use __if_freenameindex instead of
	if_freenameindex.
	(if_nameindex): Rename to __if_nameindex and define as weak alias
	of __if_nameindex.  Use libc_hidden_weak.
	(if_indextoname): Rename to __if_indextoname and define as weak
	alias of __if_indextoname.  Use libc_hidden_weak.
	* include/net/if.h [!_ISOMAC] (__if_nametoindex): Declare and use
	libc_hidden_proto.
	[!_ISOMAC] (__if_freenameindex): Likewise.
	* resolv/res_init.c (__res_vinit): Use __if_nametoindex instead of
	if_nametoindex.
	* conform/Makefile (test-xfail-XPG4/grp.h/linknamespace): Remove
	variable.
	(test-xfail-XPG4/pwd.h/linknamespace): Likewise.
	(test-xfail-UNIX98/aio.h/linknamespace): Likewise.
	(test-xfail-UNIX98/grp.h/linknamespace): Likewise.
	(test-xfail-UNIX98/pthread.h/linknamespace): Likewise.
	(test-xfail-UNIX98/pwd.h/linknamespace): Likewise.
	(test-xfail-UNIX98/sched.h/linknamespace): Likewise.
	(test-xfail-UNIX98/time.h/linknamespace): Likewise.