Add FAST encrypted challenge auth indicator

During ec_verify(), look up an authentication indicator string by the
profile realm option "encrypted_challenge_indicator".  If found, add
an indicator to the reply upon succesful creation of the challenge
key.  Add a test to t_authind.py.  Document the option in
kdc_conf.rst.

https://github.com/krb5/krb5/commit/184656dd268d3041b4fc5283ce6ddfbddfd81929
Author: Matt Rogers <mrogers@redhat.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 184656dd268d3041b4fc5283ce6ddfbddfd81929
Branch: master
 doc/admin/conf_files/kdc_conf.rst |    5 +++++
 src/include/k5-int.h              |    1 +
 src/kdc/kdc_preauth_ec.c          |   18 +++++++++++++++++-
 src/tests/gssapi/t_authind.py     |   15 +++++++++++++++
 4 files changed, 38 insertions(+), 1 deletions(-)