Stop flooding with EV_TXEMPTY and EV_RXCHAR

[Vitaliy Margolen]

Michael Karcher wrote:
> Do not generate an EV_TXEMPTY event if the transmitter is empty,
> but if it got empty. Do not generate a EV_RXCHAR event if some
> bytes are in the queue, but if some new bytes got into the queue.
> Handle a Linux quirk that TIOCGICOUNT might report characters that
> TIOCINQ does not yet report.
> 
> Add some further TRACE calls to aid debugging.

Please one change at a time (as a separate patch). In case of possible 
regressions it would be much easier to catch what was the cause.

Vitaliy.