If ftpd is run with an -h option (hide host-specific info,)
don't reveal the info in reply to the SYST command.

Get rid of using the "unix" macro at the same time. It was a rather
poor way to check if the system was Unix since there were quite a
few Unix clones out there whose cc didn't define "unix" (e.g.,
NetBSD.) It was also sensitive to the C standard used, which caused
unnecessary trouble: With -std=c99, it should have been "unix",
and so on.

PR: bin/50690
Submitted by: Alex Semenyaka <alexs _at_ snark.ratmir.ru>
MFC after: 1 week