allow passive mode range to be one port only

[tropics]

i needed this for my own server. it runs on a machine
that is behind a firewall which i can not administrate. only
two ports are usable by me, so i chose one to be the "ftp
access port" and wanted to use the other one as
"passive mode port".
looking at the server itself, i saw that it is already
possible to handle that situation. so the only thing to
change was the server interface. the patch does this and
also corrects the error message.

[tropics]

patch v1

[codesquid]

Thx, I've applied this to the repository.