Provide plugin module ordering guarantees

Rewrite the plugin internals so that modules have a well-defined
order--either the order of enable_only tags, or dynamic modules
followed by the built-in modules in order of registration.

ticket: 7665 (new)
Rely on module ordering for localauth

Register built-in localauth modules in the order we want them used by
default, and document accordingly.

ticket: 7665
Add tests for pwqual modules and plugin ordering

Create a test module for the pwqual interface, and script to exercise
the built-in and test modules through kadmin.local.  Also create a
test harness to display the order of pwqual modules for the current
configuration, and use it to test the plugin module ordering
guarantees.

ticket: 7665