Reduce the scope of the softc lock further and track device state via
sc_state.

All ucom methods apart from ucom_{read,write} are called without the
softc lock held.

ucom_close is called on last close only to match ucom_open being called
on first open only.

Fix ucom_detach where refcnt wasn't being decremented.

More DEBUG.

XXX still not sure where tty_lock needs to be held.

(skrll)