server/: Record port number with UDP socket, rather than querying.

server/: Record port number with UDP socket, rather than querying.

The old `p_port' function queried getsockname(2) at runtime, and bailed
in the (very unlikely) event that it failed.  Avoid this situation by
recording the port number when the socket is set up and just using the
recorded value when it's needed.

This was the only remaining abnormal exit after the server has started.