Add IPv6 multicast support on Linux

[iguanajuice]

partially closes #1098

[aler9]

Hello, can you confirm that this works with at least one major client (FFmpeg, GStreamer, VLC)? thanks

[iguanajuice]

I've tested udp multicast using mpv as the client and mediamtx as the rtsp server. It seems to work fine.

[aler9]

can you fix tests and everything else that was marked as failed by the CI?

[iguanajuice]

The current tests don't consider IPv6 well enough. I'm getting several "too many colons in address errors" caused by apending a colon and port number to the end of an IP address without wrapping in brackets first (e.g.: 2001:db8:0:1:2:3:4:5:8554 instead of [2001:db8:0:1:2:3:4:5]:8554)

Another problem I found in the tests is the function multicastCapableIP() returns both IP types, yet certain tests like TestServerPlayVLCMulticast assumes it's always IPv4.