Add an rc.d script to setup a netflow export via ng_netflow
The default is to export netflow data on localhost on the netflow port.
ngtee is used to have the lowest overhead possible.
The ipfw ng hook is the netflow port (it can only be numeric)
Default is netflow version 5.

Sponsored-By: Gandi.net
Reviewed by: bapt (earlier version), olivier (earlier version)