Use an opaque handle in the kdcpreauth callback

Use an opaque handle in the kdcpreauth callback

Instead of passing a request and entry to the kdcpreauth get_data
callback, pass an opaque handle.  Remove DB entry and key data
parameters from kdcpreauth methods (but keep the request, since that's
transparent).

The SecurID plugin links against libkdb5 and needs access to the client
DB entry.  Rather than continue to pass a DB entry to kdcpreauth
methods, add a get_data callback to get the client DB entry for the few
plugins which might need it.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25300 dc483132-0cff-0310-8789-dd5450dbe970