Added function "modbus_read_device_information_object"

[Schwaneberg]

This PR adds a new function to access the built in device information from Modbus devices.

Changes:

• A new function code "MODBUS_FC_READ_DEVICE_INFORMATION" was added.
• Device information object codes were added, e.g. MODBUS_OBJID_VENDORNAME.
• Function "modbus_read_device_information_object(modbus_t ctx, unsigned char object_id, char dest);" was added.
• Several internal functions were extended to handle the new function code.

Notes: I successfully tested to code with a server that uses big endian word and byte order. Therefore, the mapping in modbus.c:1432 might be wrong for clients with little endian word order, depending on how strings are handled.

Signed-off-by: Oliver Schwaneberg [email protected]

[karlp]

You need to include documentation for your new functions too

[Schwaneberg]

I will also add the documentations and update the PR.

[Schwaneberg]

I have updated the PR. Please take a look.