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

[Greg Hudson via RT]

SPAKE preauth is a great candidate for client optimistic preauth, as 
the initial support message is small and requires no credentials to 
generate.  Right now an application can request optimistic SPAKE, but 
profile configuration cannot, and it's not clear that any configuration 
should be required.