Add localauth pluggable interface

Add a new pluggable interface for local authorization, and replace the
existing krb5_aname_to_localname and krb5_kuserok implementations with
implementations based on the pluggable interface.

ticket: 7583 (new)
Add tests for localauth interface

Create a test module, program, and script to exercise the
krb5_aname_to_localname and krb5_k5userok functions as well as the
localauth pluggable interface.

ticket: 7583
Document localauth interface

ticket: 7583