tmpdir.c: Be more helpful if owner lacks permissions on the directory.

tmpdir.c: Be more helpful if owner lacks permissions on the directory.

Previously, we'd report `non-owner access permitted', which was exactly
wrong.

This is a functionality check rather than a security check, so I'm not
completely convinced that this is something tmpdir(1) should be
checking.  But it always has done so there's not much reason to stop
now.