commander: prevent potential disarms in-air
commander: prevent potential disarms in-air

This fixes the terrifying case where the drone disarms in-air just
because it receives a MAVLink disarm command. We now check param2 for a
magic number which enforces arming/disarming.

This is added to the mavlink protocol in:
https://github.com/mavlink/mavlink/pull/1162

PX4/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,dagar/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,dagar/Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware