The default was "tcp:4001", which happens to expand into
"tcp:4001:interface=127.0.0.1", which limits the listening socket to IPv4
connections only.
Changing it to "tcp:4001:interface=\:\:1" means "listen on ::1", which is the
IPv6 name for the loopback interface, and gets us a socket that accepts both
IPv4 and IPv6 connections.
Note: this might cause incompatibilities with IPv4-only hosts, if they don't
accept the "::1" name. For these systems, run the server as `twist
transitrelay --port=tcp:4001" to revert to the old behavior.
closes#12
