tmpfiles: introduce type X

tmpfiles: introduce type X

Type X will exclude path itself from clean-up. However, if the path is a
directory systemd-tmpfiles will clean-up its content.

In contrast to type x, where path is ignored completely, type X needs some
Age parameter. In order to determine Age parameter, we will look for config
entries of type d or D and pick the best match.  Best match is either
exact match or longest prefix match.