Setting the serial port speed to zero is supposed to hangup the line,
yet when I look at sys/dev/ic/com.c I see:

#if 0
	if (ospeed == 0)
		CLR(sc->sc_mcr, sc->sc_mcr_dtr);
	else
		SET(sc->sc_mcr, sc->sc_mcr_dtr);
#endif

Why has this been commented out? 
This logic could in principle be in device independent code, but
I can't see anywhere where this is done. Pointers appreciated.

Ian