Remove useless call to tcflush on open

[jannic]

Closes: #37

This is a draft PR for now, as I didn't have time to test it on real hardware.

[ijustlovemath]

I wouldn't call this useless; this is pretty standard for getting the device into a known clean state. Otherwise, you may be dropped into the middle of a protocol, which can be hard to handle.

[jannic]

I'm not saying that a flush is useless in general. But at that location it doesn't help, and it is sometimes harmful.

It doesn't help as it is called before the serial port is configured. So there is still a window where data received with mismatched settings would cause garbage in the input buffer, which is not flushed but delivered to the application.

And it may be harmful because it is done after DTR is asserted, so it may throw away actual data. (That's the issue reported in #37)

As only the application can know the right time to flush buffers, I don't think it should be done automatically during open.
This is why I added a reference to TTYPort::clear to the doc comment: It's the applications duty to throw away garbage, if necessary.

[jessebraham]

As only the application can know the right time to flush buffers, I don't think it should be done automatically during open.

Yeah, I agree with this. I did check the (not longer maintained) serial library and they are not calling tcflush() in open, so at least that's consistent.

Thanks for this!