Add IPv6 support

[Alphix]

This PR adds IPv6 support to mdns-repeater.

I've broken it out into a long list of patches which kind of track the history of rewriting the daemon. In the process I've removed some hardcoded limitations (like the number of interfaces).

I realise that it might be a bit difficult to review the individual patches. It might be better to just read through mdns-repeater.c with the whole patchset applied.