The old VN device broke in -4.x when the definition of B_PAGING
changed. This patch fixes this plus implements additional capabilities.
The new VN device can be backed by a file ( as per normal ), or it can
be directly backed by swap.
Due to dependencies in VM include files (on opt_xxx options) the new
vn device cannot be a module yet. This will be fixed in a later commit.
This commit delimitted by tags {PRE,POST}_MATT_VNDEV
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.
