Reduce redundant options in db.conf

The db.conf configuration file has a legacy that makes it look complex and confusing to new users. There is also an issue of automatically updating the file on upgrades. In practice, all current NAV subsystems log in to PostgreSQL as the same user, and the collection of "script_*" options should not be necessary. A default user should be configured with only a single entry in db.conf, and only exceptions to this rule should need a "script_xxx" option.