* Bug fix: grep -v changed to grep -iv (compare domains caselessly).
* Bug fix: nameservers now sorted in SOA serial number order, largest first.
This way you can dlint the primary server immediately after making changes
to it (previously had to wait for secondaries to do their update).
* Optimization: if any nameserver does not return an SOA record in Test 1,
it is removed from the list of nameservers and a warning is reported.
This way dlint won't use broken nameservers during the rest of the run.
* Sanity check domain names of nameservers themselves: any nameserver with
in-addr.arpa. in its name generates a warning and is skipped.
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