[Coco] drivewire serial port progress

[Aaron Wolfe]

After a long night of hacking I now have an interrupt based driver
working, and it's working really well.  Took a lot of debugging but I
learned much in the process.

I used a buffer in the style of sc6551's, I think I can get dw to
write straight into it in the future.  I somehow solved the issues
with non VDG consoles, it now works fine with level 1 or 2, any
console mode.  I also got rid of an occasional mysterious input bug.
All in all, it's coming together fast.

Performance with the interrupt driver is very good, I didn't realize
how bad my first attempt was until I got this working.
Thanks for all the help from everyone on the list.  Now I need to
implement a better set of calls between the driver and server, but
Darren sent me an awesome example of how to do this, so that won't
take too long.

It seems like some of the keyboard inputs have to be implemented in
the scf driver, for instance break/escape has to be turned into a
signal to SCF, unless I'm getting this wrong?  So it will take more
than a good terminal program to have a full featured console, I'll
have to do more work in the driver.
For data/modem applications, there is another set of controls.  Can
both be implemented at once, or will there need to be two
modes/descriptors/something?