A bit of refactoring to make future work easier.

mikeytown's patches in #652690-15: Only process CSS if the browser is IE contain some ideas to refactor this module:
- move some code out into unlimited_css_link($files, &$styles).
- move some code out into unlimited_css_import($files, &$styles).

(or better, remove the &$styles argument and use a return value instead).

I think it would make sense to do this stuff separately and before the other ideas of the above patch, so we have a common ground of discussion for future work.