* add TLSSNI01DualNetworkedServers
* use DualNetworkedServers in certbot/plugins/standalone.py
also, make both servers run on the same port.
* make probe_sni connect on ipv6 and ipv4 using None
* mimic BSD-like conditions to get test coverage
* test ServerManager taking into account BSD systems
* pass tests even if python is compiled without ipv6 support