This fixes a few problems with ide tcq, I don't know of any other known
ones (and it is solid here, survives ours of beating on it). Basically:

- Don't enable tcq if the drive isn't alone on the channel. This raises
  all sorts of fun that really requires hardware support (auto poll), or
  it's going to _suck_. I never wanted to do that, and no hardware I
  know of supports auto-poll.

- Introduce a drive blacklist so we don't enable tcq on known broken
  drives. Or enable with restrictions on some models.

- Add a check for pdc4030, apparently tcq doesn't work there (hell knows
  who would be crazy enough to pull such a stunt).