Fix getlogin_r namespace (bug 18527).

Fix getlogin_r namespace (bug 18527).

Various functions in XPG4 bring in references to getlogin_r, which is
not in XPG4; this is also a bug for some older POSIX versions which
aren't yet covered by the linknamespace tests.  This patch fixes this
by making getlogin_r into a weak alias for __getlogin_r and using
__getlogin_r as needed.

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

	[BZ #18527]
	* login/getlogin_r.c (getlogin_r): Rename to __getlogin_r and
	define as weak alias of __getlogin_r.  Use libc_hidden_weak.
	* sysdeps/mach/hurd/getlogin_r.c (getlogin_r): Likewise.
	* sysdeps/unix/getlogin_r.c (getlogin_r): Likewise.
	* sysdeps/unix/sysv/linux/getlogin_r.c (getlogin_r): Likewise.
	* include/unistd.h (__getlogin_r): Declare.  Use
	libc_hidden_proto.
	* posix/glob.c (glob): Call __getlogin_r instead of getlogin_r.
	* conform/Makefile (test-xfail-XPG3/glob.h/linknamespace): Remove
	variable.
	(test-xfail-XPG3/wordexp.h/linknamespace): Likewise.
	(test-xfail-XPG4/glob.h/linknamespace): Likewise.
	(test-xfail-XPG4/wordexp.h/linknamespace): Likewise.