lib/rules: implement forwarding

lib/rules: implement forwarding

Typical use cases should work now, briefly tested:
 - forwarding all to a resolver
 - forwarding a subtree downgraded to insecure (to resolver or auth)
 - forwarding a subtree to auth without changing DNSSEC chain (atypical)

In some places we need to repeat kr_make_query()
The issue is that some of its inputs (e.g. STUB/FORWARD, zone cut)
are now not known at the beginning of PRODUCE yet.
Also, checking that QNAME matches is useless on a cached reply.