net.Conn: Return real addresses

[edisonwsk]

How to get LocalAddr or RemoteAddr?now returns “websocket/unknown-addr ”

[nhooyr]

Right now it's hardcoded as websocket/unknown-addr but I suppose I could make it return the real addresses.

Related #331

[nhooyr]

Dealt with in #299

To the current extent possible anyway. See PR for the caveat.