Add support for krb5 1.11's responder callback.

krb5 1.11 adds support for a new method for responding to
structured data queries. This method, called the responder,
provides an alternative to the prompter interface.

This patch adds support for this method. It takes the password
and provides it via a responder instead of the prompter. In the
case of OTP authentication, it also disables the caching of
credentials (since the credentials are one-time only).