Emulate KEEP_ALIVE messages on server side.

Keep-alive messages are no longer forwarded from the front panel to the
radio. Instead they are emulated locally on the server (radio) with 150
milliseconds interval.

This change also leads to using different timeouts for the select()
calls on the server and the client.
Enable SO_REUSEADDR on server socket.

This eliminates the "Address already in use" error we otherwise
encounter when the server is restarted showrtly after it dies.