ftp: ensure client command line is available in access log

Fix that access log for FTP doors so that all responses include
the corresponding client's command line.  Previously, additional
responses (for commands that result in multi-line responses)
and all responses to certain commands (STOR and RETR) were
logged without the client's command line.

Target: master
Patch: https://rb.dcache.org/r/7607/
Acked-by: Gerd Behrmann
Acked-by: Karsten Schwank
Request: 2.11
Request: 2.10