resolved: work around broken DNS zones set up by incapdns.net

incapdns.net returns NXDOMAIN for the SOA of the zone itself but is not a
terminal. This is against the specs, but we really should be able to deal with
this.

Previously, when verifying whether an NXDOMAIN response for a SOA/NS lookup is
rightfully unsigned we'd issue a SOA lookup for the parent's domain, to derive
the state from that. If the parent SOA would get an NXDOMAIN, we'd continue
upwards, until we hit a signed top-level domain, which suggests that the domain
actually exists.

With this change whenver we need to authenticate an NXDOMAIN SOA reply, we'll
request the DS RR for the zone first, and use for validation, since that this
must be from the parent's zone, not the incorrect lower zone.

Fixes: #2894

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