Handle gateway argument to rc.opnvpn

Various fixups to make this work. Now I can:
- Unplug an interface, any OpenVPN servers/clients in a gateway group using that interface are restarted and come up on the highest tier available interface. OpenVPN servers/clients that are only on that interface go down, of course.
- Plug in the cable again, any OpenVPN servers/clients in a gateway group using that interface are restarted and come up on the now-highest tier available interface (i.e. they fail back if the interface that just came up is higher tier). OpenVPN servers/clients that are only on that interface now come up.