Detach a connection from watcher.

[liukun]

In a project, I need to detach a connection from the watcher and send it to another process, so I modified the flow of opDelete to support a new action opDetach: liukun@0a9a8f6

Do you think it can be added as a feature of gaio? If so, I'll commit a PR later with more proper designs than the above commit.

[xtaci]

but the file descriptor in the original net.Conn has been closed when watching , the working fd in watcher is dup() -ed from net.Conn.

[liukun]

Yes, it is. You can recover net.Conn by:

	f := os.NewFile(uintptr(fd), "dup-fd")
	conn, err := net.FileConn(f)
	_ = f.Close()
	return conn, err

[xtaci]

that makes sense, could you submit a PR to this issue for me to review the details?

[liukun]

Cool! I'll submit one when I have time.

不过有个设计上的问题。我目前是在回调里用size这个属性把fd传回来的。这样能接受吗？还是在OpResult里增加一个专用的FileDescriptor int或者通用的IntValue int呢？

[xtaci]

A watcher.Detach(conn) func is preferred.