Fix getpass fflush_unlocked namespace (bug 18540).

The getpass function (XPG3 / XPG4 / UNIX98) calls fflush_unlocked (not
in any of those standards).  This patch fixes this by making
fflush_unlocked into a weak alias for __fflush_unlocked and calling
__fflush_unlocked from getpass.

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

	[BZ #18540]
	* libio/iofflush.c [!_IO_MTSAFE_IO] (__fflush_unlocked): Define as
	strong alias of _IO_fflush.  Use libc_hidden_def.
	* libio/iofflush_u.c (fflush_unlocked): Rename to
	__fflush_unlocked and define as weak alias of __fflush_unlocked.
	Use libc_hidden_weak.
	* include/stdio.h (__fflush_unlocked): Declare.  Use
	libc_hidden_proto.
	* misc/getpass.c (getpass): Call __fflush_unlocked instead of
	fflush_unlocked.
	* conform/Makefile (test-xfail-UNIX98/unistd.h/linknamespace):
	Remove variable.

misc/getpass.c

@@ -91,7 +91,7 @@ getpass (prompt)
 
   /* Write the prompt.  */
   __fxprintf (out, "%s", prompt);
-  fflush_unlocked (out);
+  __fflush_unlocked (out);
 
   /* Read the password.  */
   nread = __getline (&buf, &bufsize, in);