MFC r320943-r320944, r321008, r321072, r321128

r320943:
Add ipfw_status command to etc/rc.d/ipfw

This is helpful when using service/conf management tools.

Sonsored-By: Gandi.net

r320944:
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)

r321008:
etc/rc.d: Only install ipfw_netflow is MK_IPFW and MK_NETGRAPH is defined

While here only install ipfw rc script if MK_IPFW is defined.

Reported by: ngie

r321072:
ipfw_netflow: add +ipfw_netflow_enable="NO" to defaults/rc.conf and document
usage in rc.conf(5)

Reported by: markj
Sponsored by: Gandi.net

r321128:
ipfw_netflow: Add support for FIB

If ipfw_netflow_fib, the ipfw rule will only match packets in that FIB.

While here correct some value in rc.conf(5) to be int and not str.

Sponsored by: Gandi.net