Make AS requests work with no client key

If we cannot find a client key while preparing an AS reply, give
preauth mechanisms a chance to replace the reply key before erroring
out.

https://github.com/krb5/krb5/commit/ef58e3e6f899ce60ca97bdb0053f8b912249ed0c
Author: Greg Hudson <ghudson@mit.edu>
Commit: ef58e3e6f899ce60ca97bdb0053f8b912249ed0c
Branch: master
 src/kdc/do_as_req.c   |   36 ++++++++++++++++++++----------------
 src/kdc/kdc_preauth.c |    6 ++++++
 2 files changed, 26 insertions(+), 16 deletions(-)