FR: ipv6 support for UDP trackers

[edo1]

Currently, ipv6 is only supported with HTTP trackers ("peers6" key).

https://www.bittorrent.org/beps/bep_0015.html
IPv6 announces have the same structure as v4 ones, including the used action number except that the stride size of <IP address, TCP port> pairs in the response is 18 bytes instead of 6.

As far as I understand, the current code incorrectly interprets these 18 bytes as 3 pairs of <IPv4 address, TCP port>.