UPDATING: Add an updating entry for MIT KRB5 MIT KRB5 is not default.
People may wish to play with it before it does become default. Add an
UPDATING entry to discuss the anything required to update.

Bump __FreeBSD_version to indicate that MIT KRB5 has been imported.

We will see an another __FreeBSD_version bump when MIT KRB5 becomes
the default Kerberos.