Make cross-TGT key rollover work from AD to MIT

Make cross-TGT key rollover work from AD to MIT

Active Directory always issues cross-realm tickets without a kvno,
which we see as kvno 0.  When we see that, try the highest kvno (as we
already do) and then a few preceding kvnos so that key rollover of the
AD->MIT cross TGT can work.

Add new helpers kdc_rd_ap_req, which takes the place of a couple of
steps from kdc_process_tgs_req, and find_server_key, which takes the
place of some of the end steps of kdc_get_server_key.

Code changes by Nicolas Williams.  Test cases by me.

ticket: 7109

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25799 dc483132-0cff-0310-8789-dd5450dbe970