Added -x|--xlate <port> option

 - NAT64 and other passthrough traffic may benefit
 - Redirect from in-flight traffic to 127.0.0.1:<port>
 - Use firewall rules to do this
 - This avoids any claim on the IPv4 address
 - Continue to use -l <v4listenaddr> as a publicly seen address
 - The <v4listenaddr> occurs in ICMPv6 messages as link local address