_tub_portlocation tries to detect port 0 configuration but does so incompletely

[exarkun]

For some reason configuring the main tub to listen on port 0 is not allowed:

    for port in tubport.split(","):
        if port in ("0", "tcp:0"):
            raise PortAssignmentRequired()

However this doesn't catch all possible port zero configuration strings. For example, "tcp:port=0" and "tcp:0:interface=127.0.0.1" will both also listen on port 0 and not get caught by this check.