Restore some poor D-Bus methods who got a bit hastily deleted. Enable
writing to the "last_checked_ok" property.
* mandos (ClientDBus.CheckedOK, ClientDBus.Enable,
ClientDBus.StartChecker, ClientDBus.Disable,
ClientDBus.StopChecker): Restored.
(ClientDBus.last_checked_ok_dbus_property): Make writing to this
property do
"self.checked_ok()".