Plane: reset steering integrator on mode change and when not moving

this prevents an old integrator from causing problems on takeoff

ArduPlane/Attitude.pde

@@ -364,6 +364,11 @@ static void stabilize()
         rollController.reset_I();
         pitchController.reset_I();
         yawController.reset_I();
+
+        // if moving very slowly also zero the steering integrator
+        if (gps.ground_speed() < 1) {
+            steerController.reset_I();            
+        }
     }
 }
 

ArduPlane/system.pde

@@ -390,6 +390,7 @@ static void set_mode(enum FlightMode mode)
     rollController.reset_I();
     pitchController.reset_I();
     yawController.reset_I();    
+    steerController.reset_I();    
 }
 
 // exit_mode - perform any cleanup required when leaving a flight mode