Examining structures exposed in the API

[Sam Hartman]

Love asks us to compare our API against Heimdal and see what we can do
to provide people with a way to write portable programs that access
values currently in exposed structures.  In particular, the
krb5_credentials and krb5_ticket structures are exposed.

We could use accessor functions to provide a portable API.

We actually need to think about how we are going to handle this for
extensions.