loader: Process module dependencies.

* Add string vectors for requires, optional_apis and enhances.
* Add reffed_deps module vector for holding references to dependencies.
* Initialize string vectors after final dlopen of each module.
* Free string vectors and clear references from reffed_deps in
  module_destroy.
* Create functions necessary to process module dependencies and enforce
  load order.

Module dependencies result in automatic references being managed by the
module loader.  This enforces unload order.

Change-Id: I9be08d1dd331aceadc1dcba00b804d71360b2fbb