Disable krb4 cross-realm in krb524d and krb5kdc. Provide an option to

Disable krb4 cross-realm in krb524d and krb5kdc.  Provide an option to
reenable (-X) which prints a warning that you are creating a security
hole.

Remove support for generating krb4 tickets encrypted using 3DES
service keys as it is insecure.  They are still accepted however.

The KDc is much more strict about accepting only tickets that it would
have issued in the current configuration.  In particular if the KDC
would choose some enctype for writing a TGT, other enctypes will not
be accepted when using a TGT.

Ticket: 1385
Target_Version: 1.3
Tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@15286 dc483132-0cff-0310-8789-dd5450dbe970