ftp: add logging to access log file

Add support for logging FTP interactions to the access log.  Each
client command is logged along with the corresponding reply from
dCache.  Base64-encoded context-handling and user's passwords are
not logged.  Encoded commands and replies are indicated as
encrypted and the plain-text version is logged.

Requires-notes: yes
Requires-book: yes
Target: master
Patch: http://rb.dcache.org/r/6584/
Acked-by: Gerd Behrmann