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;

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -845,6 +845,28 @@ MulticopterPositionControl::start_flight_task()
 			// we want to be in this mode, reset the failure count
 			_task_failure_count = 0;
 		}
+
+	} else if (_vehicle_status.nav_state == vehicle_status_s::NAVIGATION_STATE_DESCEND) {
+
+		// Emergency descend
+		should_disable_task = false;
+		FlightTaskError error = FlightTaskError::NoError;
+
+		error =  _flight_tasks.switchTask(FlightTaskIndex::Descend);
+
+		if (error != FlightTaskError::NoError) {
+			if (prev_failure_count == 0) {
+				PX4_WARN("Descend activation failed with error: %s", _flight_tasks.errorToString(error));
+			}
+
+			task_failure = true;
+			_task_failure_count++;
+
+		} else {
+			// we want to be in this mode, reset the failure count
+			_task_failure_count = 0;
+		}
+
 	}
 
 	// manual position control

src/modules/navigator/navigator_main.cpp

@@ -591,11 +591,6 @@ Navigator::run()
 			_precland.set_mode(PrecLandMode::Required);
 			break;
 
-		case vehicle_status_s::NAVIGATION_STATE_DESCEND:
-			_pos_sp_triplet_published_invalid_once = false;
-			navigation_mode_new = &_land;
-			break;
-
 		case vehicle_status_s::NAVIGATION_STATE_AUTO_RTGS:
 			_pos_sp_triplet_published_invalid_once = false;
 			navigation_mode_new = &_dataLinkLoss;
@@ -620,6 +615,7 @@ Navigator::run()
 		case vehicle_status_s::NAVIGATION_STATE_ACRO:
 		case vehicle_status_s::NAVIGATION_STATE_ALTCTL:
 		case vehicle_status_s::NAVIGATION_STATE_POSCTL:
+		case vehicle_status_s::NAVIGATION_STATE_DESCEND:
 		case vehicle_status_s::NAVIGATION_STATE_TERMINATION:
 		case vehicle_status_s::NAVIGATION_STATE_OFFBOARD:
 		case vehicle_status_s::NAVIGATION_STATE_STAB: