Remove the bulging firewalls section from the security chapter into a
new chapter all its own, currently in the 'network communications'
part as chapter 24.

Add a small paragraph at the bottom of the security chapter synopsis
noting that additional security topics are covered in the MAC chapter
and the Firewall chapter.

While here add some missing punctuation, cleanup the whitespace, add
missing </para> tags, add a synopsis, and of course
s/<sectN/<sectN-1/.