Remove the #error that temporarily stopped this from supporting slices and

Remove the #error that temporarily stopped this from supporting slices and
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.