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.

test/units/testsuite-74.mount.sh

@@ -149,3 +149,9 @@ touch "$WORK_DIR/mnt/foo/baz"
 systemd-umount "$WORK_DIR/mnt/foo"
 test -d "$WORK_DIR/mnt/foo/bar"
 test ! -e "$WORK_DIR/mnt/foo/baz"
+
+# overlay
+systemd-mount --type=overlay --options="lowerdir=/etc,upperdir=$WORK_DIR/upper,workdir=$WORK_DIR/work" /etc "$WORK_DIR/overlay"
+touch "$WORK_DIR/overlay/foo"
+test -e "$WORK_DIR/upper/foo"
+systemd-umount "$WORK_DIR/overlay"