Rework of firewall chapter start

[David Adam]

Simon,

>-    <para>All software-based firewalls provide some way to filter
>+    <para>Firewalls makes it possbile to filter

Should be "Firewalls make it possible to filter".

I'm not really sure where the need to remove all references to firewalls
being software (for example, "OpenBSD PF firewall *software*") came from.

Cheers,

David Adam
zanchey at ucc.gu.uwa.edu.au