ftp: fix help output

The HELP command was added to allow better interoperability with
ncftp and Globus clients.  Unfortunately, the output for 'SITE HELP'
and 'HELP SITE' was incorrect.

While ncftp proved robust against this bug (it wasn't detected
during previous testing) and Globus code is likely OK too, I
request back-porting so dCache output conforms with RFC 959.

Target: master
Patch: https://rb.dcache.org/r/7575/
Acked-by: Gerd Behrmann
Request: 2.11
Request: 2.10
Request: 2.9