Copter: disarm after completing RTL

Note that an RTL command executed in AUTO mode will also disarm when it
lands and pilot's throttle is put to zero and no further commands will
be executed.  This is normally not an issue because missions generally
end with an RTL (instead of having the RTL in the middle) and a work
around is available in that the LAND command could be used instead of
RTL.

ArduCopter/control_rtl.pde

@@ -42,7 +42,7 @@ static void rtl_run()
             // do nothing
             break;
         case Land:
-            // do nothing
+            // do nothing - rtl_land_run will take care of disarming motors
             break;
         }
     }
@@ -278,6 +278,18 @@ static void rtl_land_run()
 
     // check if we've completed this stage of RTL
     rtl_state_complete = ap.land_complete;
+
+#if LAND_REQUIRE_MIN_THROTTLE_TO_DISARM == ENABLED
+    // disarm when the landing detector says we've landed and throttle is at minimum
+    if (ap.land_complete && (g.rc_3.control_in == 0 || failsafe.radio)) {
+        init_disarm_motors();
+    }
+#else
+    // disarm when the landing detector says we've landed
+    if (ap.land_complete) {
+        init_disarm_motors();
+    }
+#endif
 }
 
 // get_RTL_alt - return altitude which vehicle should return home at