disk infrastructure changes.

[Matt Thomas <matt%3am-software.com@localhost>]

I put a patch at http://www.netbsd.org/~matt/ncq-disk-diffs.txt

It includes a set of related changes:

1) make disklabel handling in the disk driver more common.
   Also allow variable sized disklabel ioctls removing compat code.

2) move replicated softc flags into struct disk.

3) NCQ support.  Almost, ahcisata/siisata still need to increase the number of 
openings
   to fully enable it.  This requires moving things from the ata_channel to the 
ata_xfer
   and being able to handle more than one outstanding xfer at a time.

I'm not a disk/storage person but these changes do work as little as I've 
tested them.
feedback is welcome.