[krbdev.mit.edu #8537] Preauthentication should continue after failure

[Greg Hudson via RT]

Upon receiving a KDC_ERR_PREAUTH_REQUIRED error, the client consults the 
module for each padata type in the METHOD-DATA until one succeeds.  
After that point, pre-authentication could fail on the KDC side with a 
KDC_ERR_PREAUTH_FAILED error, or on the client side after a 
KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error.  In either of those cases, we 
should go back to processing the original METHOD-DATA list (or the one 
in the KDC_ERR_PREAUTH_FAILED message if one was supplied), skipping the 
failed mechanism.

(We also do not continue after failure resulting from optimistic 
preauth, unless it happens immediately on the client side.  That problem 
is out of scope for this ticket.)