modbus plugin: Add documentation and rename to "datagroup".

modbus plugin: Add documentation and rename to "datagroup".

Florian,

Attached is the updated version of the datagroup patch. The code itself didn't
change (apart from the rename dataset -> datagroup). It should apply cleanly
to the latest version of the master branch.

I also attached a patch to the configure.in script to make pkg-config check
for 'libmodbus' as well as 'modbus'. Different versions of the library use
different names.

Greetings,

Ivo De Decker

Signed-off-by: Florian Forster <octo@huhu.verplant.org>