validate: work around some SERVFAILs (iterating mode)

- This affects the iterating mode only (except for a verbose message).
- The problem was introduced when implementing forwarding in 651c5aad.
- Example that was affected: *.org.ru.

When the same server is authoritative for multiple consecutive zones,
with a higher one being signed and a lower one being unsigned, we may
get an unsigned answer even in case we *think* we're currently in a
signed zone.

In particular, DS (sub-)queries are important because of being used to
prove the insecurity of zones - in that case the iterator can correctly
determine the (unexpected) zone name from which the answer came, due to
seeing the SOA record, but that is too late for zone_cut_check() to try
getting a trust chain to it, so we need to YIELD for it to do it.