Only writing commands must be allowed in broadcast mode

[stephane]

By reading again, https://github.com/stephane/libmodbus/blob/master/tests/unit-test-client.c#L497
I see the test isn't good because only write operations are allowed in broadcast mode

Extract from http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf page 7/44:

No response is returned to broadcast requests sent by the master. The broadcast requests are necessarily writing commands. All devices must accept the broadcast for writing function. The address 0 is reserved to identify a broadcast exchange.

[jcwren]

Is this a pretty easy fix? It'd be great to see this get in there.