slapd.postinst should put all backed-up items together in one place under /var/backups

Currently the slapd.postinst script uses /var/backups/slapd-<old-package-version>/ to store both the backup copy of $SLAPD_CONF and the "slapcat"-generated .ldif file. However, if there is a need to move the BDB files out of the way, they are instead moved to separate <rootdn>-<old-packge-version>.ldapdb destination directories found directly under /var/backups/ .

Assuming the move_incompatible_databases_away logic is kept in post-Lucid, it might be worth changing the destination path so that it keeps the BDB files together with the other backups under /var/backups/slapd-<old-package-version>/ .

(I think that could be accomplished by simply changing the hard-coded "/var/backups" strings found in the compute_backup_path and move_incompatible_databases_away functions to instead call database_dumping_destdir, similar to the call in backup_config_once .)