navigator: do not set a mode while disarmed
navigator: do not set a mode while disarmed

This avoids premature state machine execution, most notably for missions.

In particular this fixes these issues:
- when landed, and the vehicle has a valid mission: switching to mission
  would execute the first item.
- after executing a mission with a land waypoint, the mission is reset
  and the first item is executed.

In case the first waypoint is a takeoff, it would print a user message:
"Takeoff to x meters above home.". If it was a vehicle command, it would
execute that.
Note that because the vehicle is disarmed, the vehicle would not actually
do anything (except for commands that are accepted while disarmed).

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