Reuseable ports

Add an option to make the local port reusable for other sockets. This enables scenarios where excon is used to assist in NAT peer discovery. An example usage: https://github.com/mikehale/rendezvous-socket/blob/use-excon/lib/rendezvous/socket.rb#L52.

Depends on https://github.com/geemus/excon/pull/351