[krbdev.mit.edu #6430] If we fail to generate preauth, don't loop

[Greg Hudson via RT]

I reverted Luke's fix on trunk and then fixed the PREAUTH_FAILED looping
bug as Sam described in a separate ticket (#6573, r22890).

I am leaving this bug report open as Sam's original description is still
true--that is, we will loop 16 times trying un-preauthenticated AS-REQs
if we can't do any of the preauth mechanisms presented to us by the
server in a PREAUTH_REQUIRED error.