Mech/Train sensor

If enabled, detects mechs/trains and blocks their movement if they are coming from any of the directions set by dir. Can currently only be enabled/disabled by radio command and airlocks are updated to utilize this.

Currently allows mechs to move through airlocks during normal cycling, which for example lets Odysseus out to save lives, but once a shuttle docks the sensors are enabled to prevent boarding.

If blocked, players receive a simple message stating that their command has been overridden and they remain in place in front of the sensor turf.