Support listening with HAProxy "proxy protocol"

The proxy protocol allows forwarding a client's real address from a reverse proxy to the application at the TCP layer (i.e. without the reverse proxy having to break open TLS). There's an implementation of this in hyper_server that could probably be used (or stolen).

We're not actually using source IP addresses for anything at the moment (we're using into_make_service() rather than into_make_service_with_connect_info()).