Demand for US-made durable goods remained flat in May, with higher orders for airplanes and defense goods offsetting weaker sales of machinery and metals, according to Commerce Department figures released today.   Excluding demand for transportation equipment (which rose 2.6 percent), orders for durable goods fell 0.9 percent, the first drop in three months. Defense orders were up 10.9 percent, but after taking them away, total orders fell 0.6 percent.