Simplify the ioctl handling in GEOM.

Simplify the ioctl handling in GEOM.

This replaces the current ioctl processing with a direct call path
from geom_dev() where the ioctl arrives (from SPECFS) to any directly
connected GEOM class.

The inverse of the above is no longer supported.  This is the
situation were you have one or more intervening GEOM classes, for
instance a BSDlabel on top of a MBR or PC98.  If you want to issue
MBR or PC98 specific ioctls, you will need to issue them on a MBR
or PC98 providers.

This paves the way for inviting CD's, FD's and other special cases
inside GEOM.