Add the `pdu_type` field to Modbus over TCP logs to indicate whether the Modbus
message was a request or a response. Due to the client/server nature of Modbus
over TCP/IP, all messages from the TCP session originator are requests, while
all messages from the TCP session responder are responses.
Adding this information to the default log surfaces protocol metadata in a way
that doesn't require users to understand the Modbus over TCP protocol.
For modbus message types that include variable amount of register values
(uint16[]), setting a &length attribute without an explicit array size
could trigger a parsing assertion since it allows for the "element" data
pointer to travel past the "end of data" (e.g. when &length is odd).
This is changed to now give both an array size and &length to earlier
terminate the parsing of elements before the assert is checked and
so a single out-of-bound check can be done for the entire array
(leaving off &length causes an out-of-bound check for each element).
Added another parameter to modbus events that carry register arrays to
the script-layer which indicates the associated byte count from the
message (allowing for invalid values to be detected):
modbus_read_holding_registers_response
modbus_read_input_registers_response
modbus_write_multiple_registers_request
modbus_read_write_multiple_registers_request
modbus_read_write_multiple_registers_response
modbus_read_fifo_queue_response
