hurd: Fix dirfd symbol exposition from ftw

dirfd is XOPEN2K8 only, it should not be exposed along ftw which is earlier.

	* include/dirent.h (__dirfd): New declaration.
	* dirent/dirfd.c (dirfd): Rename to __dirfd, and redefine as weak
	alias.
	* sysdeps/posix/dirfd/dirfd.c (dirfd): Likewise.
	* sysdeps/mach/hurd/dirfd.c (dirfd): Likewise.
	* io/ftw.c (open_dir_stream, ftw_dir): Use __dirfd instead of dirfd.