Copter: GPS failsafe ensures LAND is pilot-controlled

This resolves issue #880 in which if a GPS failsafe occurred while the
vehicle was already in LAND mode, the LAND would continue to be GPS
controlled LAND instead of pilot-controlled LAND

ArduCopter/control_land.pde

@@ -172,3 +172,11 @@ static bool update_land_detector()
     // return current state of landing
     return ap.land_complete;
 }
+
+// land_do_not_use_GPS - forces land-mode to not use the GPS but instead rely on pilot input for roll and pitch
+//  called during GPS failsafe to ensure that if we were already in LAND mode that we do not use the GPS
+//  has no effect if we are not already in LAND mode
+static bool land_do_not_use_GPS()
+{
+    land_with_gps = false;
+}

ArduCopter/events.pde

@@ -215,6 +215,11 @@ static void failsafe_gps_check()
             set_mode(LAND);
         }
     }
+
+    // if flight mode is LAND ensure it's not the GPS controlled LAND
+    if (control_mode == LAND) {
+        land_do_not_use_GPS();
+    }
 }
 
 // failsafe_gps_off_event - actions to take when GPS contact is restored