Authenticate the multiparty key if available

 * Warning: breaks backwards compatibility.

 * If there's a mp key for the user, add it to the secret when
   performing the smp. Detect if that key changes (or suddenly becomes
   available after authentication) and warn the user.

 * This definitely needs review.
Refactor smp answer preparation

 * Also, a more descriptive warning func name
Merge pull request #599 from arlolra/mp

Authenticate the multiparty key if available