UARTs when their divisor latch registers are selected while they are
doing output. Waiting for (some) output to drain is not permitted for
the TCSANOW case of tcsetattr(). NIST-PCTS easily detects bugs like
this by testing at at speeds that no one would want to use (50 bps).
Deleted stale comments related to flushing i/o. Flushing works properly
for 16550s according to NIST-PCTS (it can't work properly for 16450s).
This finishes fixing all sio(16550)-related bugs found by NIST-PCTS.
8280a33470