adding relay port

[shoebox639]

Currently, the relay port is always random. This doesn't work well if the server is dockerized. It needs to know which ports to forward when the server starts. It looks like the server already takes in relay port, but the client is just never sending it.

This PR adds a -r, --relay-port option to the client CLI. If the option is set, that port will be sent to the server and used as the relay port.