FlightTasks: add Descend task to land without GPS

This adds a flight task to catch the case where we want to do an
emergency descent without GPS but only a baro.

Previously, this would lead to the navigator land class being called
without position estimates which then made the flight tasks fail and
react with a flight task failsafe. This however meant that landed was
never detected and a couple of confusing error messages.

This applies if NAV_RCL_ACT is set to 3 "land".

src/modules/commander/Commander.cpp

@@ -3197,8 +3197,7 @@ Commander::update_control_mode()
 		break;
 
 	case vehicle_status_s::NAVIGATION_STATE_DESCEND:
-		/* TODO: check if this makes sense */
-		control_mode.flag_control_auto_enabled = true;
+		control_mode.flag_control_auto_enabled = false;
 		control_mode.flag_control_rates_enabled = true;
 		control_mode.flag_control_attitude_enabled = true;
 		control_mode.flag_control_climb_rate_enabled = true;