enc_padata can include empty sequence

enc_padata can include empty sequence

There are two issues with return_enc_padata.
1)  It often will return an empty sequence of enc_padata rather than not including the field
2) FAST negotiation is double supported in the referral tgs path and not supported in the non-referral path

Rewrite the return_enc_padata logic to:

* Split  out referral interactions with kdb into its own function
* Use add_pa_data_element

ticket: 6656
target_version: 1.8
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23712 dc483132-0cff-0310-8789-dd5450dbe970