Implement 32-bit keytab kvno extension

Heimdal and Shishi support a 32-bit kvno at the end of a keytab entry,
overriding the 8-bit version if present.  Implement this in the FILE
keytab type and document it in keytab_file_format.rst.

https://github.com/krb5/krb5/commit/54b4ccf510b67140caec76b12fff4b30462e7e50
Author: Greg Hudson <ghudson@mit.edu>
Commit: 54b4ccf510b67140caec76b12fff4b30462e7e50
Branch: master
 doc/formats/keytab_file_format.rst |   10 +++++-----
 src/lib/krb5/keytab/kt_file.c      |   26 +++++++++++++++++++++++++-
 2 files changed, 30 insertions(+), 6 deletions(-)