Recognise servers which are dual-stack.

Requires a master server with support for the ‘dual’ flag.

Server querying is modified to work with this.
IP v4/v6/any preference setting.

Controls whether Osavul will prefer IPv4 or IPv6 for contacting servers and
for launcing Unvanquished.

Of course, if you select ‘prefer IPv6’ and you don't have any public IPv6
addresses configured, well, you're not going to get very far.
Missing #include, fixing a build failure.
Update copyright notice.