Peter M. Groen / libmodbus

Thanks to @peternewman

- more coherent
- namespace
- opaque and smaller context
- usual wording

The ID used at init time will be the device ID of the caller and
the server ID in request functions is the target to reach.

- adjusts various comments on return values
- updated tests
- added entry to MIGRATION

- MAJOR changes to the API (see MIGRATION)
- New unit tests
- Removed TODO list
- Updated NEWS & MIGRATION files

This change improves the conformance with the PDU and ADU definitions of the
Modbus protocol and reduces the number of additions.

- new functions modbus_slave_init_listen_tcp, modbus_slave_accept_tcp,
  modbus_slave_slave_receive.
- removed printf in modbus.c: 'Connection closed'
- new slave test, bandwith-slave-many-up
- updated build scripts
- updated MIGRATION document file