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

This makes it easier to detect unexpected errors.

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.

And upgrade dependencies.

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.

The dovecot-keylookupd is a dict-proxy dovecot lookup daemon, which
can read public and private keys from the LDAP backend and use them
for Dovecot userdb / passdb lookups.