Peter M. Groen / libmodbus

Fix EXXX values defined in Microsoft C Runtime 10

Thanks to Bernhard Agthe.

- avoid export of _modbus_rtu_set_rts and change its name to avoid
  confusion
- namespace RTU functions and constants
- use_rts is renamed rts
- add missing '_' in function name
- remove useless debug message in usual write
- wrong message about 'not supported'
- add check for TIOCM_RTS support
- block RTS flow control when not available

bit is used HIGH to able to transmit, or LOW.

Avoid incompatibility with GPLv2 program. This change has been approved by
Tobias Doerffel, Florian octo Forster and Hannu Vuolasaho.

modbus_set_slave must be used to set the slave ID of the remote
device to talk in master mode and to set the internal slave ID in
slave mode.

If you talk to several devices, you need to call modbus_set_slave
each time the following requests must be sent to another device.

This change fixes a compilation problem in MinGW. As general rule,
each header must be as independent as possible.

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