Returns the port number the parser uses for user connections.
If no argument is given, the port for this_player() is
returned. If this_player() is not existing or not interactive,
the first port number open for connections is returned.
If an user object is given, the port used for its connection
is returned.
If a positive number is given, the <num>th port number the
parser uses for connections is returned (given that there are
that many ports).
If -1 is given, the number of ports open for connections is
returned.