Add a hack (ported from NetBSD) to support Sun disk labels.

Add a hack (ported from NetBSD) to support Sun disk labels.
This code works by converting the Sun label to a struct disklabel, which
is probably even the right thing for reading a label. The original
checksum is taken over, so that the label source can be distinguished.

The NetBSD code to wrap a BSD-style disklabel into the Sun disklabel has been
deleted for now - don't know whether that is really desirable, after all Sun
disklabels could just be used always (BSD disklabels are going to have
problems with PROM compatability).  The dsinit() call in diskopen() has been
#ifdef'ed out for now, this will be changed to use the minimal slice struct
in case of dsinit() failure.

Submitted by:	tmm
Obtained from:	NetBSD