uhid(4): Use d_cfdriver/devtounit/cancel to avoid open/detach races.

- Split uhidclose into separate uhidcancel and uhidclose parts.
  uhidcancel interrupts pending I/O operations (open, read, write,
  ioctl, &c.); uhidclose doesn't run until all I/O operations are
  done.

- Handle case where, owing to revoke(2), uhidcancel/uhidclose run
  concurrently with a uhidopen that hasn't yet noticed that there
  isn't actually a device.

- Handle case where, owing to revoke(2), uhidread might be cancelled
  by mere revoke, not by detach, so it has to wake up when the device
  is closing, not (just) when dying (but dying will lead to closing
  so no need to check for dying).

- Omit needless reference-counting goo.  vdevgone takes care of this
  for us by cancelling all I/O operations with uhidcancel, waiting
  for I/O operations to drain, closing the device, and waiting until
  it is closed if that is already happening concurrently.

- Name the closed/changing/open states rather than using 0/1/2.

- Omit needless sc_dying.

(riastradh)