Add RTU compatibility to DUE

[NitrofMtl]

With use of a modified ArduinoRs485 lib, make use modbus with Arduino DUE.

Change, add macro: -In modbus.c: -change order of inclusion -remove inclusion that did not prevent compiling ( look like already included in arduino.h) -remove the #include <unistd.h>, it did not compile with it, but no problem seen without

-in modbus-rtu.cpp: -change the param for Serial.begin from uint16_t to UARTClass::UARTModes