The messages should look less confusing now, as the phrasing for
expected errors has been changed.

This makes it easier to detect unexpected errors.

Add SQL support

See merge request !1

Introduces the "sql" backend, and refactors backends behind a nicer
interface (which looks a lot like the one in id/auth). Users can
define arbitrary SQL queries to fetch the key data, to adapt to their
database schema.

Include vendored dependencies for sqlite3/mysql/postgres.

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.