Allow to choose the RTS activation level for RS485

[epsilonrt]

Hello Stephane,
I created mbpoll (https://github.com/epsilonrt/mbpoll) to provide an open source tool for controlling MODBUS slaves.
This program uses libmodbus. It has some success, especially on ARM platforms (but it is multiplatform).
Under certain circumstances, it is necessary to control the signal DE of the RS485 drivers in the low state. It depends on the electronic structure of the RS485 part.
So I modified your code to add this possibility.
I now wish to propose mbpoll as a debian package and I must of course separate your code from mine.
I propose you this modification hoping that you will agree.
Thank you.
Best Regards

[karlp]

At the very least, you'll need to update the documentation as well to go with this.

[epsilonrt]

Okay, I take care of it at the end of the week and I modify the documentation in this way. Thanks.