Use type-safe callbacks in preauth interface

Use type-safe callbacks in preauth interface

Replace the generic get_data functions in clpreauth and kdcpreauth
with structures containing callback functions.  Each structure has a
minor version number to allow adding new callbacks.

For simplicity, the new fast armor key callbacks return aliases, which
is how we would supply the armor key as a function parameter.  The new
client keys callback is paired with a free_keys callback to reduce the
amount of cleanup code needed in modules.

ticket: 6971

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25315 dc483132-0cff-0310-8789-dd5450dbe970