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

[Greg Hudson via RT]

All that code does is sort specified preauth types to the front of the
list.  It doesn't add preauth types to a nonexistent list.  (It's also
an undocumented parameter; I will try to fix that.)

Anyway, I believe we agree.  I will change the code to only continue on
PREAUTH_FAILED during the first iteration.  Eliminating the separate
dispatches in the preauth framework would be a great bit of cleanup, but
will take more time than I have available on the margin.