vnconfig doesn't seem to be in Lite2, although dev/vn is. The Lite2
merge in userland has converted all mount calls except this one and
one for devfs in init, so we can drop support for old mount args soon.
Vnconfig probably shouldn't support mounting - it only supports ufs.
This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
