Peter M. Groen / libmodbus

Thanks to Lars Taeuber.

Warning, modbus_receive returns 0 when a query is ignored.

- function removed from TCP and RTU code and from backend structure
- updated documentation
- updated examples

These new functions seem clearer than the recent modbus_receive_from
function IHMO. This change fixes the bandwidth-server-many-up program.

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