This makes conn.logs a bit prettier (and smaller) because all lines that
do not use a tunnel will now have a "-" instead of the "(empty)" for
tunnel_parents.
- Fix parsing of empty question sections (when QDCOUNT == 0). In this
case, the DNS parser would extract two 2-byte fields for use in either
"dns_query_reply" or "dns_rejected" events (dependent on value of
RCODE) as qclass and qtype parameters. This is not correct, because
such fields don't actually exist in the DNS message format when
QDCOUNT is 0. As a result, these events are no longer raised when
there's an empty question section. Scripts that depends on checking
for an empty question section can do that in the "dns_message" event.
- Add a new "dns_unknown_reply" event, for when Bro does not know how
to fully parse a particular resource record type. This helps fix a
problem in the default DNS scripts where the logic to complete
request-reply pair matching doesn't work because it's waiting on more
RR events to complete the reply. i.e. it expects ANCOUNT number of
dns_*_reply events and will wait until it gets that many before
completing a request-reply pair and logging it to dns.log. This could
cause bogus replies to match a previous request if they happen to
share a DNS transaction ID.
