Only use one sending socket; auto-determine ports

Only use one sending socket; auto-determine ports

For sending UDP packets, we only need one socket to be
open - both types of packets (input and state) can be
sent through the same socket.

Simplify the configuration of multi-player by setting
the pre-defined ports based on which machine is the
master and which one is the slave. This way, we only
have to specify for each instance the type (slave or
master) and the hostname/ip of the opponent.