实现在指定目录及其子目录中搜索指定扩展名的文件，并更新为新的扩展名的程序reext

程序功能：

　　1．程序带有三个命令行参数dir、old和new，用法为：

　　reext dir old new

　　其中Dir是一个全路径名，old和new是文件扩展名，如：

　　Reext c:\ASM\TEST txt doc

　　2．程序实现的功能为：在路径Dir中搜索文件扩展名为txt的所有文件，如果找到，将其扩展名改为doc，而文件名不变：

　　3．较高要求，可以搜索指定目录的全部子目录，这需要递归程序实现。

-Achieve the specified directory and its subdirectories in the search for the file name extension, and to update the extension for the new program features reext procedure: 1. Procedures with the three command-line parameters dir, old and new, use for: reext dir old new one of Dir is a full path name, old and new are file extensions, such as: Reext c: \ ASM \ TEST txt doc 2. Procedures for the implementation of the function: in the path of Dir in the search for a file extension of all the txt file, if found, will be replaced by the extension doc, and the file name the same: 3. Higher requirements, you can search all the subdirectories of the specified directory, which required implementation recursive procedures.