showing another GCS in vehicle list

[tridge]

If there are multiple GCS on the radio link with different sysid then MissionPlanner can end up showing the other GCS in the vehicle list: in this example sysid 251 is a mavproxy instance that was forwarding to MissionPlanner This led to some confusion as MissionPlanner defaulted to the mavproxy GCS as a vehicle, and showed the copter flight modes for a quadplane. We should be able to exclude GCS instances using HEARTBEAT MAV_TYPE of GCS