FreeBSD/doc 17bfa0b (r52936)en_US.ISO8859-1/books/handbook/firewalls chapter.xml

This patch updates the PF chapter regarding structure and various updates.

After discussing some updates to the PF chapter with current maintainer kp@,
I came up with the following list of changes in this patch:

- Change the warning message to remove the reference to a specific PF version
and point out that FreeBSDs pf has diverged from what's in OpenBSD.
- Remove a link to the old pf porting efforts, which is obsolete nowadays and
doesn't provide much value to the reader anymore
- Change the reading flow by putting the ALTQ section at the end of the chapter.
ALTQ with PF is not used by everyone, so having that at the end of the chapter
is good to not overwhelm the reader too much with concepts they probably don't
need.
- Remove the "In order to" sentence parts which are mostly superfluous and can
be replaced with "to" for increased readability
- Sprinkle in some sysrc in examples where it makes sense to use it

Reviewed by:    kp@
Event:          Aberdeen hackathon 2019
Differential Revision:  https://reviews.freebsd.org/D19939
DeltaFile
+121-123en_US.ISO8859-1/books/handbook/firewalls/chapter.xml
+121-1231 files

UnifiedSplitRaw