Fix a Net::DNS version quirk in the "resolver" system test

Net::DNS versions older than 0.68 insert a ./ANY RR into the QUESTION
section if the latter is empty.  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/resolver/ans8/ans.pl
should behave consistently for various Net::DNS versions.  Ensure that
by making handleUDP() return the query ID and flags generated by
Net::DNS with 8 zero bytes appended.

(cherry picked from commit 6c3c6aea)