Ticket #2841: ftpfs: new filelist parser.

Initial step: minor refactoring:
  * (vfs_parse_month): rename from is_month() and make global.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
NOT TESTED: Add ftp long file list parser from lftp

...and modify it for MC VFS.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
NOT_TESTED: ftpfs: use new parser.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>