Implement DNS64 synthesis.

Implement DNS64 synthesis.
When unwind(8) learns new autoconf resolvers (from dhcp or router
advertisements) it checks if a DNS64 is present in this network
location and tries to recover the IPv6 prefix used according to
RFC7050.
The learned autoconf resolvers are then prevented from upgrading to
the validating state since DNS64 breaks DNSSEC.
unwind(8) can now perform its own synthesis. If a query for a AAAA
record results in no answer we re-send the query for A and if that
leads to an answer we synthesize an AAAA answer using the learned
prefixes.

Testing & OK kn