check-config: add config option checker This script scans files for lines that look like either ui.config usage or config variable documentation. It then ensures: - ui.config calls for each option agree on types and defaults - every option appears to be mentioned in documentation It doesn't complain about devel/experimental options and allows marking options that are not intended to be public. Since we haven't been able to come up with a good scheme for documenting config options at point of use, this will help close the loop of making sure all options that should be documented are.