* lib/model: Don't panic on failed chmod-back on directory (fixes#5836)
This makes the "in writable dir"-wrapper log chmod-back errors instead
of panicking. To do that we need a logger so the function moved into the
model package which is also the only place it's used. The tests came
along.
(The test also exercised osutil.RenameOrCopy like some sort of
piggybacking. I removed that.)
