Modify the bitstream loading routine to work directly with the
Raw Binary Files (.rbf) generated by Altera tools. Previously,
a custom format was used which was basically an RBF preceded by
a 4-byte header specifying the transfer length.
(lwla_send_bitstream): Unref the mapped file earlier in order
to simplify the error handling.
(lwla_receive_reply): Do not treat a reply buffer length of
zero as silent no-op. That logic was left over from an earlier
iteration, before the distinction between reply buffer size and
expected read length was introduced.