unused for now, but I plan to convert all programs in base to use it in
a future diff. /dev/bpf0 is for compatibility with existing binaries
and is to be removed after a transition period.
ok rpe krw, for the installer part
"Let's see it hit the tree." deraadt

In effect, this removes the "old" vndX nodes, and renames
the svndX nodes to vndX.
Old svndX nodes will still continue to work though, for now.
Cleanup accordingly.
ok deraadt@, todd@
comments and ok on the man page bits from jmc@