IPv6-only mode for hosts behind CGNAT

[Knogle]

Ahoy, i hope you are doing fine.

Right at the moment i got downgraded to CGNAT and IPv6 because our new carrier has no publicly available IPv4 addresses for customer connections.
So as i workaround, and probably the feature, IPv6 only would be a nice measure.

Implemented here, IPv4 works quite fine for the clients as well, but as noted, clients have to connect through the IPv6 instead due to CGNAT.

#1287