ucom(4): Rework open/close/attach/detach logic.

- Defer sleep after hangup until open.

  No need to make close hang; we just need to make sure some time has
  passed before we next try to open.

  This changes the wchan for the sleep.  Oh well.

- Use .d_cfdriver/devtounit/cancel to resolve races between attach,
  detach, open, close, and revoke.

- Use a separate .sc_closing flag instead of a UCOM_CLOSING state.
  ucomcancel/ucomclose owns this flag, and it may be set in any state
  (except UCOM_DEAD).  UCOM_OPENING remains owned by ucomopen, which
  might be interrupted by cancel/close.

- Rework error branches in ucomopen.  Much simpler this way.

- Nix unnecessary reference counting.

(riastradh)