MDNS: Support multiple simultaneous interfaces

[eskimor]

While this pull handles changes in the default interfaces well, it does not yet support multiple interfaces at the same time. This can be handled by using platform specific functions for specifying the outgoing interface for sending messages. By default, the implementation will pick the default interface for multicast messages, which works fine if there is only one interface.

[cryptoquick]

Would this be helpful if, say, I wanted peer discovery on a VPN, say, using https://github.com/dswd/vpncloud ? It sets up a separate network interface device, under /sys/class/net on Linux.

[eskimor]

I believe so - yes.

[cryptoquick]

That was the one thing I cratered on last I tried. This'd be a big help.

[dvc94ch]

this one can be closed