Simplify admin settings form

I think the settings form can be simplified. There isn't really a need for collapsible fieldsets, as each contains only one option. The explanatory text is repeated for each content type, but is only seen when a fieldset is expanded.

This patch is modeled after the settings for About this node. It provides a bit of help text at the top, then a simple list of checkboxes.

The variables are stored differently in this version, but the old ones persist in the database. If this patch is accepted, then addanother.install might be needed to clean up the cruft and transfer settings to the new variable.

The patch is against v6.x-1.1