Continue I/O board interface improvements:

- Remove the "txnotfull" signal that would wake up an API caller
  if the buffer was full.  This should never happen and indicates
  that we are feeding the hardware faster than it can handle.
  Blocking in the caller is disastrous; just don't do it.

- Change the update thread to bundle all transmits together.
  If, for example, 5 packets are ready to be sent, first transmit
  all 5 (in one system call, the flush is not done until after
  the final transmit), then wait for all 5 responses.  This
  improves parallelism.

- Remove arbitrary 50usec delay between send and receive.

- Limit 8 lamp outputs per IOB for now, since the Arduino code can't
  process 32 yet.

- Shorten the RDG commands sent, since the debounce flag isn't used.