Net::DNS versions older than 0.67 respond to queries sent to a
Net::DNS::Nameserver even if its ReplyHandler returns undef.  This makes
the "serve-stale" system test fail as it takes advantage of the newer
behavior.  Since the latest Net::DNS version available with stock
RHEL/CentOS 6 packages is 0.65 and we officially support that operating
system, bin/tests/system/serve-stale/ans2/ans.pl should behave
consistently for various Net::DNS versions.  Ensure that by reworking it
so that it does not use Net::DNS::Nameserver.

(cherry picked from commit c4209418)