core: create workdir/upperdir when mounting a Type=overlay mount unit

core: create workdir/upperdir when mounting a Type=overlay mount unit

So far we created the target directory, and the source for bind mounts,
but not workdir/upperdir for overlays, so it has to be done separately
and strictly before the unit is started, which is annoying. Check the
options when creating directories, and if upper/work directories are
specified, create them.