Update Changelog for 2.1beta2 release
App-layer: Add Modbus protocol parser

Decode Modbus request and response messages, and extracts
MODBUS Application Protocol header and the code function.

In case of read/write function, extracts message contents
(read/write address, quantity, count, data to write).

Links request and response messages in a transaction according to
Transaction Identifier (transaction management based on DNS source code).

MODBUS Messaging on TCP/IP Implementation Guide V1.0b
(http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf)
MODBUS Application Protocol Specification V1.1b3
(http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf)

Based on DNS source code.

Signed-off-by: David DIALLO <diallo@et.esia.fr>
Detect: Add Modbus keyword management

Add the modbus.function and subfunction) keywords for public function match in rules (Modbus layer).
Matching based on code function, and if necessary, sub-function code
or based on category (assigned, unassigned, public, user or reserved)
and negation is permitted.

Add the modbus.access keyword for read/write Modbus function match in rules (Modbus layer).
Matching based on access type (read or write),
and/or function type (discretes, coils, input or holding)
and, if necessary, read or write address access,
and, if necessary, value to write.
For address and value matching, "<", ">" and "<>" is permitted.

Based on TLS source code and file size source code (address and value matching).

Signed-off-by: David DIALLO <diallo@et.esia.fr>
Detect-engine: Add Modbus detection engine

Management of Modbus Tx

Based on DNS source code.

Signed-off-by: David DIALLO <diallo@et.esia.fr>