Static cache clearing for key API functions

I ran into a few issues while writing unit tests for Features & ImageCache integration:

- imagecache_presets($reset = FALSE) does not pass on $reset to imagecache_preset_actions(). This means you may get stale static cached actions on the presets returned.
- imagecache_preset_actions($preset, $reset = FALSE) does not properly clear its static cache.
- imagecache_preset_by_name() has an unclearable static cache.

The attached patch addresses these issues.