First pass support for detecting using avahi

This is missing correct handling for IPv6 (IPv6 is now in), and has not added any tests nor changed the packaging.

I'm not sure yet how best to write tests for this, and this shouldn't be merged until the IPV6 fix is in, but I figure I should post this in case there's some issue with the approach.

Fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1051206