[krbdev.mit.edu #8656] Implement client optimistic SPAKE, perhaps by default

[Greg Hudson via RT]

Optimistic SPAKE might not be completely safe in scenarios like the 
following: a KDC offers PKINIT and 1FA SPAKE for a principal, the 
former providing either a more convenient passwordless login experience 
or a more generous authentication indicator, the latter being a 
fallback option for devices that don't possess the client certificate.  
If the client does optimistic SPAKE, it won't learn about the KDC's 
offer of PKINIT, and will ask for the password and/or settle for not 
getting the PKINIT auth indicator.

So perhaps explicit configuration should be required for the client to 
use optimistic SPAKE.