说明：  ftp的多线程下载和单线程上传的实现。利用本程序，一方面可以理解ftp指令，另一方面可以深入理解多线程
(The multi-threaded ftp download and upload the realization of single-threaded. The use of this procedure can be understood on the one hand, ftp command, on the other hand to make a thorough understanding of multi-threaded)