Incorporate Apple's patch

Incorporate Apple's patch

Add a test authorization data scheme, in both built-in and plugin
forms; built-in version is #ifdef'ed out.  Update configury to create
the build directory for the plugin, but don't build or install it by
default.

Create the new (and normally empty) authorization data plugin
directory at install time.

Add some (normally disabled) code to log authz data from rd_req.

Fix up some comments that still refer to preauth plugins.  Add some
details in comments on the API, and why it's private for now.

Make the plugin init context support work, by not passing null
pointers.

ticket: 5565

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@20691 dc483132-0cff-0310-8789-dd5450dbe970