systemd: serialize/deserialize forbid_restart value

systemd: serialize/deserialize forbid_restart value

The Service type's forbid_restart field was not preserved by
serialization/deserialization, so the fact that the service should not
be restarted after stopping was lost.

If a systemctl stop foo command has been given, but the foo service
has not yet stopped, and then the systemctl --system daemon-reload was
given, then when the foo service eventually stopped, systemd would
restart it.

https://bugs.freedesktop.org/show_bug.cgi?id=69800