Concerns about the Response_set interface

[Sam Hartman]

I missed a significant concern. I think the suggestions I made may
already help manage this concern.

Many of the credential acquisition applications support both MIT
Kerberos and Heimdal.
We need to minimize  future areas where credential acquisition gets in
the way of portability.

the current interface seems like it lends itself to
implementation-specific data structures  that will make application
portability challenging.