Continue untangling the disklabel. Reorganize struct partinfo and the

Continue untangling the disklabel.  Reorganize struct partinfo and the
DIOCGPART ioctl to extract the required information directly, and fix
the DIOCGPART ioctl direction so userland can use it.

This removes numerous disklabel references, particularly from the filesystem
code which was doing silly indirections just to figure out the sector size.

NOTE: The absolute byte offset of the slice or partition (relative to the
base of the raw disk) is also made available, but is not currently used
by the kernel.