Fixup gwlb functions related to gw failover

Make apinger pass the gateway parameter both when an alarm alarms (gateway is down) and when it comes good (gateway is up). This way the downstream openvpn and dyndns code can know what happened and do sensible stuff.
Return the friendly interface name ("wan", "opt1" etc) rather than the actual device name ("vr0", "vr1" etc). The openvpn and dynamic dns code that uses this needs to work with various entries in the config that all use the friendly interface name.
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.
Merge pull request #625 from phil-davis/master

Fixup handling gateway argument to rc.openvpn