Don't require themes to completely override preprocess functions

I've found that if a module's preprocess functions use the module name instead of "template" any theme that wishes to add or change things with a preprocess function must fully re-implement the preprocess function. Additionally, the re-implementation can be unreliable since on cache clears both preprocess functions can run. If the preprocess functions are named with template all the variables will be set and handed off to the overriding preprocess function where the values can be changed, added or removed (i.e. template_preprocess_privatemsg_view in favor of privatemsg_preprocess_privatemsg_view). This greatly reduces the amount of code that needs to be copied around and simplifies things for themers who may not understand or be interested in the logic that's being added by the module.

For what it's worth, this is how it's suggested to implement a module's base preprocess functions: http://drupal.org/node/223430

I've attached a patch that does this for privatemsg and have confirmed it cleans up a lot of our custom privatemsg theme issues.