Listen on IPv6 if called with -6

I found that http-replicator didn't listen on IPv6 even if called with
-6 (that apparently only makes it use IPv6 on outgoing
connections). This patch changes the call to socket.socket to use
AF_INET6 if -6 is used.  I also had to fix the message printed for
accepted connections.

I'm not certain that this is the best way to do it, but it works for
me. ;)