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

ok 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@