This commit prevents most non-Modbus TCP traffic on port 502 to be
reported as Modbus in conn.log as well as in modbus.log.
To do so, we have introduced two &enforce checks in the Modbus
protocol definition that checks that some specific fields of the
(supposedly) Modbus header are compatible with values specified in
the specs.
To ensure non-regression, with this commit we also introduce a
new btest.
Closes#3962
