For multiple interfaces on one sub-net, advertise the address with the preferred route metric

[cmcqueen]

Eg a machine might have Ethernet and Wi-Fi both connected to the same sub-net. In the routing table, the machine will prefer to use the interface with the lower value for "metric" (most likely Ethernet).

It would be ideal for avahi to advertise the address that is on the preferred interface for routing.

On the machine I'm currently using (an embedded Linux device) with both Ethernet and Wi-Fi, avahi is advertising the Wi-Fi IP address, but it would be more sensible to advertise the Ethernet address.

If it's too hard for avahi to work it out from the routing table (which I suppose might be true for more complex routing tables), then it would be good to have a config option to specify interface preference order for this scenario.