Add IPv6 support

FDF should support IPv6 multicast protocols (SSDP, mDNS, etc.).

I don't currently use IPv6 on my home network (other than the always-present link local addresses), as my current ISP does not support it.  Thus, I don't really have a good way to test it.

Please add a comment to this issue if you have a use for IPv6 support and are interested in testing.