reset slow_WP option at mode switch

ArduCopter/system.pde

@@ -419,6 +419,9 @@ static void set_mode(byte mode)
 	// clearing value used to force the copter down in landing mode
 	landing_boost = 0;
 
+	// do we want to come to a stop or pass a WP?
+	slow_wp = false;
+
 	// do not auto_land if we are leaving RTL
 	auto_land_timer = 0;