modbus_receive - receive an indication request
The modbus_receive() function shall receive an indication request from the
socket of the context
ctx. This function is used by a Modbus slave/server to
receive and analyze indication request sent by the masters/clients.
If you need to use another socket or file descriptor than the one defined in the
ctx, see the function modbus_set_socket.
The function shall store the indication request in
req and return the request
length if successful. The returned request length can be zero if the indication
request is ignored (eg. a query for another slave in RTU mode). Otherwise it
shall return -1 and set errno.