Support for dual stack IPv4 and IPv6 environments

[cr]

In dual stack environments, Foxbox and client won't necessarily be using the same external IP address for server requests. Foxbox should ensure to register through both IPv4 and IPv6 where available.

It might make sense to return both the internal IPv4 and IPv6 address regardless of which stack was used by the client for the ping. The addresses could be coalesced by a strong random (but reproducible) ID used by the Foxbox for registration.

cloud.desre.org doesn't support IPv6 at this point, so this is certainly a low-priority issue.