Why not hook "sleep" and "usleep"?

As poll has been hooked, they can be easily implemented.

After hooked them, we can simply write period "consumer" program （offline scanner）.

因为poll已经被挂钩，所以sleep和usleep可以轻松的用poll来实现协程式的休眠。
如果  sleep和usleep被挂钩，我们可以轻松的实现周期性工作的消费者程序（在离线扫描程序中很常见）。