Track first local TGT key in KDC code

Track first local TGT key in KDC code

Decrypt the first local TGT key in get_local_tgt() and save it in the
AS and TGS processing functions.  (As we now sort key data by
descending kvno, this is guaranteed to be the most recent key.)  Pass
this key to the authdata and FAST cookie functions to simplify cookie
encryption and authdata signing.  Decryption and verification
functions must still sometimes decrypt earlier keys to process tickets
predating the last local TGT key rollover.