networkctl: introduce "persistent-storage" command

networkctl: introduce "persistent-storage" command

Then, this introduces systemd-networkd-persistent-storage.service.

systemd-networkd.service is an early starting service. So, at the time
it is started, the persistent storage for the service may not be ready,
and we cannot use StateDirectory=systemd/network in
systemd-networkd.service.

The newly added systemd-networkd-persistent-storage.service creates the
state directory for networkd, and notify systemd-networkd that the
directory is usable.