Fix scandir scandirat namespace (bug 17999).

The POSIX function scandir calls scandirat, which is not a POSIX
function.  This patch fixes this by making it use __scandirat and
making scandirat a weak alias.  There are no changes for scandir64 /
scandirat64 because those are both _GNU_SOURCE-only functions so no
namespace issue arises for them.

Tested for x86_64 that the disassembly of installed shared libraries
is unchanged by this patch.

	[BZ #17999]
	* dirent/scandir.c [!SCANDIR] (SCANDIRAT): Define to __scandirat
	instead of scandirat.
	* dirent/scandirat.c [!SCANDIRAT] (SCANDIRAT): Likewise.
	[!SCANDIRAT] (SCANDIRAT_WEAK_ALIAS): Define.
	[SCANDIRAT_WEAK_ALIAS] (scandirat): Define as weak alias of
	__scandirat.
	* include/dirent.h (scandirat): Do not use libc_hidden_proto.
	(__scandirat): Declare.  Use libc_hidden_proto.
	* conform/Makefile (test-xfail-POSIX2008/dirent.h/linknamespace):
	Remove variable.
	(test-xfail-XOPEN2K8/dirent.h/linknamespace): Likewise.