Try to be smarter generating invitation URLs.

If we have a symbol hostname associated with us, use that. Otherwise, try
to find out which external addresses we have for both IPv4 and IPv6. If we
cannot connect to www.meshlink.io, then use the local address used for
outgoing connections as a fallback. Also, don't forget to add a port number
in all cases.

The generated invitation URL can now contain two addresses in the hostname
part, separated by a comma.