resolved: when caching negative responses, honour NSEC/NSEC3 TTLs

When storing negative responses, clamp the SOA minimum TTL (as suggested
by RFC2308) to the TTL of the NSEC/NSEC3 RRs we used to prove
non-existance, if it there is any.

This is necessary since otherwise an attacker might put together a faked
negative response for one of our question including a high-ttl SOA RR
for any parent zone, and we'd use trust the TTL.

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