Properly scope per-request preauth data

Properly scope per-request preauth data

It should be possible to successfully use multiple initial credentials
contexts with the same library context.  Create a new internal type
krb5_preauth_req_context containing per-request preauth state,
including the clpreauth modreq handles and the list of preauth types
already tried.  Remove this state from clpreauth_handle and
krb5_preauth_context.

ticket: 7877