directory from /var/backup (useful for those of us who have a separate /var
and would like to have our backup disklabels on the root filesystem).
Default behaviour unchanged. backup_dir being unset is taken as /var/backup.
consistent with what FreeBSD uses /etc/defaults for and since SVR4
uses /etc/default for another purpose. as discussed on tech-userlevel,
and no objections were made.