Allow to process special FTP-Commands (like OPTS, FEAT ...) at start of data transfer

New Option "pre_ftp_comands": a List of "pure" FTP-Commands, separatet by a colon ";", which are understandable directly by the server

Example:
OPTS UTF8 ON
FEAT

see SOSFTP-78 for using this option together with the processing of UTF-8 encoded filenames.