Make yes/no options checkboxes, not two radios

It's so obvious now…

This doesn't seem to be that much superior after all:
- the problem with the checkboxes is that Drupal styles them very differently … http://skitch.com/wimleers/2ikj/form-inconsistencies
- the bold strings indicate the settings. And it's also how they're named internally. So if you're a developer, it'll allow you to quickly find the settings you're looking for.

For now, I'll keep it this way. Drupal core does the same thing at admin/settings/performance. Plus, I'd have to update the JS and I don't have much time now.

Tagging for HS4. Included in the HS4 roadmap: http://drupal.org/node/1052670.

This will happen in the 7.x branch.