Addig support for using a pre-defined GPIO port for RTU RTS.

Hi Stephane,

this extension can be useful on small ARM boards like Raspberry PI, Olinuxino, BeagleBones and so on. These boards usually have many GPIO ports to use as digital input/output, but not eventually having support for controlling RTS through ioctl calls or just the RTS pin overlaps with another port, reserved for some reasons. By specifying the directory where the dedicated GPIO port was mapped to the filesystem, libmodbus can use this port to controll the RS-485 IC's DE/RE pins.

Please feedback to me, if there are things to fix. I could test it successfully on BeagleBone Black, using LTC485 IC to communicate with a Dixell XR40CX thermostat.

Best regards, and thank you for this great library!
Aron Papp