* rename PART to DISKLABEL
* allow to take an OFFSET range to make it possible to access an image
where there is no disklabel present
* make future extensions easier and less intrusive for the callers
systems should be mounted with ukfs_mount_disk() from now on. The
partition argument specifies which label is being mounted (or the
entire image). E.g. partition 4 should be label 'e'.