Return INSECURE when validating DNS replies which have RRSIGs, but
when a needed DS record in the trust chain is proved not to exist.
It's allowed for a zone to set up DNSKEY and RRSIG records first, then
add a DS later, completing the chain of trust.

Also, since we don't have the infrastructure to track that these
non-validated replies have RRSIGS, don't cache them, so we don't
provide answers with missing RRSIGS from the cache.

  If multiple routes to the same network exist, Linux blindly picks
  the first interface (route) based on destination address, which might not be
  the one we're actually offering leases. Rather than relying on this,
  always set the interface for outgoing unicast DHCP packets.

The nasty code with static variable in retry_send() which
avoids looping forever needs to be called on success of the syscall,
to reset the static variable.