Avoid implementing our own version of asp/userenckey/u2f serialization
and deserialization.

Sets 'noauthenticate', and prefixes the private key with the userdb_
prefix as it should.

And upgrade dependencies.

The field is prefixed by an ID in LDAP (separated from the key data by
a ':'), strip it before returning key data from the backend.

And make HTTP result codes slightly more meaningful.

This allows Dovecot to fail temporarily if the key lookup backend is
unavailable or has issues talking to LDAP.