Adding flow control to CC65 up2400 driver.

Am barrelling through PLATOTerm, and I've found that checking the receive
buffer in the main-line code and asserting flow control there doesn't seem
to be 100% effective in dealing with what still looks like some buffer
overrun.

Given this code here:
https://github.com/nanoflite/c64-up2400-cc65

Would adding a buffer overflow check and asserting bit 1 of $DD01 to assert
hardware flow control work? I'm asking because I know this code is _very_
timing sensitive, and I am not very familiar with the user port serial
machinery.