specifies the source directory for the users and user file. It can be
specified as $SPOOLDIR or "${SPOOLDIR-/var/spool/gnuspool}" etc and the
environment and/or gnuspool.conf will be interrogated to
interpolate the value of the environment variable given.
If you use this, don't forget to put single quotes around it thus:
gspl-spuconv -D '${SPOOLDIR-/var/spool/gnuspool}' ...
otherwise the shell will try to interpret the $ construct and not
gspl-spuconv.