Plane: implement VTOL landing for AFS termination

this allows for vertical landing as an AFS_TERM_ACTION

ArduPlane/afs_plane.cpp

@@ -16,6 +16,12 @@ AP_AdvancedFailsafe_Plane::AP_AdvancedFailsafe_Plane(AP_Mission &_mission) :
  */
 void AP_AdvancedFailsafe_Plane::terminate_vehicle(void)
 {
+    if (plane.quadplane.available() && _terminate_action == TERMINATE_ACTION_LAND) {
+        // perform a VTOL landing
+        plane.set_mode(plane.mode_qland, MODE_REASON_FENCE_BREACH);
+        return;
+    }
+
     plane.g2.servo_channels.disable_passthrough(true);
     
     if (_terminate_action == TERMINATE_ACTION_LAND) {

ArduPlane/failsafe.cpp

@@ -86,8 +86,10 @@ void Plane::failsafe_check(void)
 #if ADVANCED_FAILSAFE == ENABLED
         if (afs.should_crash_vehicle()) {
             afs.terminate_vehicle();
+            if (!afs.terminating_vehicle_via_landing()) {
                 return;
             }
+        }
 #endif
 
         // setup secondary output channels that do have

ArduPlane/quadplane.cpp

@@ -1653,7 +1653,7 @@ void QuadPlane::update(void)
     }
 
 #if ADVANCED_FAILSAFE == ENABLED
-    if (plane.afs.should_crash_vehicle()) {
+    if (plane.afs.should_crash_vehicle() && !plane.afs.terminating_vehicle_via_landing()) {
         motors->set_desired_spool_state(AP_Motors::DesiredSpoolState::SHUT_DOWN);
         motors->output();
         return;
@@ -1845,7 +1845,9 @@ void QuadPlane::motors_output(bool run_rate_controller)
     }
 
 #if ADVANCED_FAILSAFE == ENABLED
-    if (!hal.util->get_soft_armed() || plane.afs.should_crash_vehicle() || SRV_Channels::get_emergency_stop()) {
+    if (!hal.util->get_soft_armed() ||
+        (plane.afs.should_crash_vehicle() && !plane.afs.terminating_vehicle_via_landing()) ||
+         SRV_Channels::get_emergency_stop()) {
 #else
     if (!hal.util->get_soft_armed() || SRV_Channels::get_emergency_stop()) {
 #endif

ArduPlane/servos.cpp

@@ -672,8 +672,10 @@ void Plane::set_servos(void)
 #if ADVANCED_FAILSAFE == ENABLED
     if (afs.should_crash_vehicle()) {
         afs.terminate_vehicle();
+        if (!afs.terminating_vehicle_via_landing()) {
             return;
         }
+    }
 #endif
 
     // do any transition updates for quadplane