Add support for Sonos Discovery packets

Merge support for setting TTL (although actually this should not be required since we don't decrement the TTL when relaying us performed).

Add support for relaying broadcast packets between interfaces.

Add relaying of broadcast UDP packets on port 6969 to support Sonos Discovery (i.e. the mechanism where by the app can find new speakers when you push the "connect" button on the back of the speaker).