Fix handling of session key for Kerberos5. I don't think this should

Fix handling of session key for Kerberos5.  I don't think this should
fix the mutual authentication bug with beta 4, but this should help
forwarding credentials and should also help if someone actually
defines ENCRYPTION.

If user defines ENCRYPTION, telnetd has the nasty behavior of
encrypting one side of the conversation, without telling the user; in
particular, only server output is encrypted.  Fixed by sending a do
encrypt if encryption defined; fix taken from Athena telnetd.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@6955 dc483132-0cff-0310-8789-dd5450dbe970