Allow sysop-choice of source of virtual sub-directory source name

By default, each file transfer directory's internal code suffix is
used as the source of the sub-directory of the virtual path used
to represent a file area in the FTP and Web servers. Now, a sysop
can change that source to either each directory's short name or
long name, if they prefer.

For MRO, because he asked (on DOVE-Net).