fix(net): Fix TCP Unresponsiveness and Inability to Close Connections (#791)

* fix(net): Improve stability. 为RawSocket与UdpSocket实现close时调用close方法，符合smoltcp的行为。为SocketInode实现drop，保证程序任何情况下退出时都能正确close对应socket, 释放被占用的端口。

* fix(net): Correct socket close behavior.

user/apps/http_server/main.c

@@ -233,6 +233,8 @@ int main(int argc, char const *argv[])
         // 关闭客户端连接
         close(new_socket);
     }
+    // 关闭tcp socket
+    close(server_fd);
 
     return 0;
 }