Support MKDIR and RMDIR commands for ProFTPd

**FTP OS:** Unix 

**FTP Server:** ProFTPD

Now that we support detection of ProFTPd via the ServerType, we should support MKDIR and RMDIR internally to speed up those operations on ProFTPd.

This issue has more info: https://github.com/robinrodricks/FluentFTP/issues/88#issuecomment-304430823