Add DIR command - not sure if/where this was documented before

Update the description of the GET and PUT commands too.

text/menu/sysxfer.asc

                         Synchronet Sysop Transfer Menu
                      (All commands are preceeded by a ';')
 
-PUT [s]         Direct upload file s to any drive or directory (remote)
-GET [s]         Direct download file s from any drive or directory (remote)
+DIR [path]      List directory contents of current directory or [path]
+PUT [file]      Direct upload [ile to any drive or directory (remote)
+GET [file]      Direct download file from any drive or directory (remote)
 OLD           *	Remove, Edit, or Move files not downloaded since new-scan date
 OLDUL         *	Remove, Edit, or Move files uploaded before new-scan date
 UPLOAD        *	Bulk local upload - add files on disk to database