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