Project review: responder (round 2)

[ghudson@MIT.EDU]

Nathaniel has updated the project proposal and candidate patch for the
responder callback based on the previous round of feedback.  The
project proposal is at:

    http://k5wiki.kerberos.org/wiki/Projects/Responder

and the candidate patch is at:

    https://github.com/npmccallum/krb5/tree/responder

Instead of binary contracts, questions now have a "challenge" and an
"answer".  (The term "question" is generally used to indicate the name
of a question, in the current terminology.)  Challenges and answers
are required to be printable UTF-8 text; complicated data is expected
to be marshalled using JSON.

In the previous round of feedback, Sam requested that as an API
convenience, we support preemptive answers (similar to how you can
provide a password to krb5_get_init_creds_password() instead of
supplying the password through the prompter).  The current proposal
doesn't implement preemptive answers, but isn't incompatible with it.