UMCR_DTR
SET(combits, UMCR_DTR);
CLR(sc->sc_mcr, UMCR_DTR | UMCR_RTS);
if (how == TIOCMSET || ISSET(combits, UMCR_DTR))
ucom_dtr(sc, (mcr & UMCR_DTR) != 0);
if (ISSET(combits, UMCR_DTR))