Several fixes to ftp_list plugin

Fixes to ftp_list plugin especially related to :
 - size computation and folder detection when MLST is not supported by the ftp server
 - file path definition when a path with multiple subdirectories is used in the plugin configuration
 - management of options "recursive" and "files-only" (some cases were ignored)
 - handling of spaces in directories names

Please note that I was not able to test the modifications on a FTP server that supports MLST but everything should work correctly.