[pf4freebsd] Minor changes to the port

[Max Laier]

Hello,

"Chris" found some minor issues with the port. I incooperated his comments
and you'll find the new version on the webpage:

1) The sample pf.conf is now installed to ${PREFIX}/etc/pf.conf.default in
order not to overwrite a users modified version.
2) pf.sh is installed as pf.sh.sample as well and pfctl is now called with
complete pathname (${PREFIX}/sbin/pfctl) instead of pfctl alone.

He further asked:

> 1) Something about needing Options PFIL_HOOKS and
> Options RANDOM_IP_ID compilied into the kernel for pf
> when using make for the port.
>
> 2) Needing to add: gateway_enable="YES" to
> /etc/rc.conf if you wish to have this machine act has
> a gateway.

I personally think, that 1) is covered by pkg-descr and that this is enough.
What's your opinon?
2) is a rather generic issue and nothing special about pf. I don't think we
need to mention.

If one would write a small step-by-step tutorial on getting pf operational
after installing from the port, I'd love to include that to the PORTSDOC

Thanks for your feedback
    Max