labels.
Please test the slice/label features:
cd /dev; sh MAKEDEV svn0
cd /var/tmp; dd if=/dev/zero of=vnfile bs=8192 count=1024
vnconfig -c /dev/rvn0 /var/tmp/vnfile
fdisk /dev/rvn0 # invent a geometry, create one BSD partition
disklabel -r -w vn0 floppy # a convenient (bogus) label
disklabel -e vn0 # edit label to match device
newfs /dev/rvn0a
mount /dev/vn0a /mnt
...
The steps after the fdisk could name the device vn0s1 instead of vn0.
is supposed to work except "media removal" (shutting down the vn driver)
while some partitions are open.
Fix some errnos: return ENOTTY, not ENXIO for unknown ioctls; return
ENODEV, not ENXIO for the unsupported dump operation.
