Allow type.inc files to add their our module settings

As suggested on http://ken.therickards.com/2006/10/20/mysite/#comments by Carol Clarke.

It makes sense to let the type.inc files define specific settings that apply to the type. In the core code, there are aggregator-specific settings that are only active if the aggregator module is present.

The original feature request asked for restrictions on taxonomies. That is best handled by term.inc.

This would also mean moving the aggregator settings into feed.inc and/or aggregator.inc.