Add an rc.d script to start pfsync at the right moment of the
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