IPv6 multicast and unicast support

[dfn8979]

Hi, I have tried setting an IPv6 address as a value of "UnicastIPv6" (instead of the "UnicastIPv4" from the README) in the Options registry key. Unfortunately, this does not seem to work - no IPv6 packets related to Scream appear on the network interface. The Unix/Linux receiver also doesn't accept an IPv6 address as the value of -i:
$ ./scream -u -p 4011 -i 2001:db8::11
Invalid interface: 2001:db8::11

There are also no IPv6 examples and there's no information about IPv6 support in the README, which makes me believe that Scream currently does not support IPv6, which prevents it from working on IPv6-only hosts.

It's important for software like Scream to support IPv6-only networks, even software that will be used primarily on internal LANs, because IPv4 RFC1918 space is limited and prone to conflicts (for instance when using various types of VPNs) - many organisations (or even people with more complex home networks) use IPv6 exclusively to get around that.