Peter M. Groen / libmodbus

These functions have no meaning in RTU so it's better to specialize
the names and remove them from the backend.

- remove the functions from the backend
- update tests to handle RTU mode (master and slave)
- add command line options to tests (rtu or tcp)

- move termios header in RTU backend
- move TCP headers in TCP backend
- use native serial on Win32 for Cygwin
- avoid too many defines

Added support for native Win32 based on
https://code.launchpad.net/~thepyper/libmodbus/win32-native
This mainly affects the RTU implementation as the TCP/IP socket
interface is the same thanks to the winsocks library. The interface
for using the serial port is completely different on Win32 and is now
implemented accordingly.

Signed-off-by: Stéphane Raimbault <stephane.raimbault@gmail.com>

All private functions and constants are now prefixed by _.
The modbus protocol uses entry points to call specific functions.