Adding ad[4-6]s* to default devices

[Chris Turner]

Any thoughts?

More systems are coming out with 2 controllers - a PATA and a SATA,
and as a result there might be more users using ad4 and so on..

I know there's the MAKEDEV.local hook, just thought I'd mention it since
I keep having having to remember to MAKEDEV on some my systems after
installing them. If OK I'll probably go ahead and make the change.

Thanks,
- Chris