Create the vn%d.ctl devices.

In a few days I will commit a patch which changes vn(4) to use the
disk-minilayer. This will make vn(4) fully DEVFS friendly but have
the side effect that vnconfig needs the vn%d.ctl devices to be able
to configure vn(4).

Please remake your /dev/vn entries with this revision of MAKEDEV if
you don't rung DEVFS already.