gen_tcp 应对对端半关闭

当tcp对端调用shutdown(RD/WR) 时候， 宿主进程默认将收到{tcp_closed, Socket}消息， 如果这个行为不是你想要的，那么请看：

shutdown(Socket, How) -> ok | {error, Reason}

Types:

Socket = socket()
How = read | write | read_write
Reason = posix()

Immediately close a socket in one or two directions.

How == write means closing the socket for writing, reading from it is still possible.

To be able to handle that the peer has done a shutdown on the write side, the {exit_on_close, false} option is useful.

这样就不会被强制退出了。