Make _dbus_directory_get_next_file use readdir, not readdir_r
This isn't thread-safe or reentrant, but it turns out we don't need
either of those properties, and readdir_r is a real pain to use correctly,
particularly in the presence of FUSE filesystems that might implement
statfs() wrong.
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=8284
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=15922 Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>