Copter: skip mode change checks when disarmed

Note: there will be a follow up commit to move the checks into a
separate function so they can be called just before arming
Merge remote-tracking branch 'diydrones/master'
Rover: support new RADIO_STATUS msg
Copter: support new RADIO_STATUS msg
Plane: support new RADIO_STATUS msg
AP_HAL: add nullptr macro (next step towards c++11)
Merge remote-tracking branch 'diydrones/master'