The point of setting TMUX_TMPDIR is to then make any labels from -L go to
that directory. In the case of makesocketpath() with no TMUX_TMPDIR set,
would set both the path and the default socket to a file. The checking of
the permissions on the file worked fine in that case, but when TMUX_TMPDIR
is set, won't work on a directory.
This fixes the problem by ensuring the check on the permissions is performed
on directories only.