Add a new dns_rdataset_equals() function to check whether two
rdatasets are equal in DNSSEC terms.
When an rdataset being cached is rejected because its trust
level is lower than the existing rdataset, we now check to see
whether the rejected data was identical to the existing data.
This allows us to cache a potentially useful RRSIG when handling
CD=1 queries, while still rejecting RRSIGs that would definitely
have resulted in a validation failure.
(cherry picked from commit 6aba56ae89)
Evan Hunt
b2e11b1ad3