Add an API-specific lookup method to DNSBackend.

Add an API-specific lookup method to DNSBackend.

This method, APILookup(), behaves similarly to lookup() but allows
disabled records to be returned to the caller. Backends with no support
for disabled records (bind, geoip, ldap, lua2, pipe, tinydns) implement
it as a by-default wrapper over lookup(). Other backends override with
their own processing.

SQL-style backends use distinct queries, api-id-query and
api-any-id-query, so as not to penalize non-API workloads.