Plane: use MIS_AUTORESET when entering AUTO mode

ArduPlane/system.pde

@@ -342,12 +342,8 @@ static void set_mode(enum FlightMode mode)
     case AUTO:
         auto_throttle_mode = true;
         prev_WP_loc = current_loc;
-        // start the mission. Note that we use resume(), not start(),
-        // as the correct behaviour for plane when entering auto is to
-        // continue the mission. If the pilot wants to restart the
-        // mission they need to either use RST_MISSION_CH or change
-        // waypoint number to 0
-        mission.resume();
+        // start or resume the mission, based on MIS_AUTORESET
+        mission.start_or_resume();
         break;
 
     case RTL: