Add kdcpreauth callback to check for client keys

Add a new have_client_keys callback to the kdcpreauth interface,
allowing modules to efficiently check whether the client DB entry has
any keys matching the request enctypes.

https://github.com/krb5/krb5/commit/e50482720a805ecd8c160e4a8f4a846e6327dca2
Author: Greg Hudson <ghudson@mit.edu>
Commit: e50482720a805ecd8c160e4a8f4a846e6327dca2
Branch: master
 src/include/krb5/kdcpreauth_plugin.h |    8 ++++++++
 src/kdc/kdc_preauth.c                |   20 ++++++++++++++++++--
 2 files changed, 26 insertions(+), 2 deletions(-)