While running a test suite of a DNS server, a lot of DNS messages on
non-standard ports were not recognized. Rather than manually discovering
and decoding every port using an iterative process of checking the
output of the `udp and not dns` filter, have some heuristics to detect
DNS messages automatically.
Enable these heuristics by default assuming that the checks are strong
enough, 8 bytes are essentially fixed to a low number of possibilities.
Should it cause issued, then the heuristics could be disabled (assuming
that non-standard DNS ports are uncommon) or strengthened.
