system boot, and hook it up in the system.
The separate script is needed because in the presence of various
interface lists in rc.conf ($network_interfaces, $cloned_interfaces,
$sppp_interfaces, $gif_interfaces, more to come) it is hard to start
them orderly, so that pfsync is brought up after its syncdev, which
is required for the proper startup of pfsync.
Discussed with: mlaier on -pf
MFC after: 5 days
