ticket: 6700

ticket: 6700
subject: Introduce new krb5_tkt_creds API

Merged from branches/iakerb: add new asynchronous krb5_tkt_creds APIs,
which allow a caller to take responsibility for transporting requests
to the KDC and getting responses back.  Rewrite the existing
krb5_get_credentials API in terms of the new functions.  Get rid of
krb5_get_cred_from_kdc and friends, since they are no longer used.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23900 dc483132-0cff-0310-8789-dd5450dbe970