Peter M. Groen / libmodbus

Thanks to @peternewman

- protect against NULL IP address BTW
- update documentation

- update documentation
- 5 new tests
- updated NEWS file
- avoid include of time.h

Add macros in libmodbus.txt and:
- modbus_get_byte_from_bits.txt
- modbus_get_float.txt
- modbus_reply.txt
- modbus_reply_exception.txt
- modbus_set_bits_from_byte.txt
- modbus_set_bits_from_bytes.txt
- modbus_set_float.txt

The function name was confusing because the write operation is performed
before the read. Take care to swap the arguments in the migration process.

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

All related scripts and config files have been updated too.

Split the original modbus_receive function in two functions to avoid
the strange -1 value to ignore the sockfd argument.