Update chapter 14 "Security" for Debian 10

Update the chapter
- Take into the account the feedback received in #24 (closed) (about the apparmor section)
- Reconsider the fwbuilder recommendation (or put a disclaimer/work around => using iptables-legacy in the mean time) in light of https://github.com/fwbuilder/fwbuilder/issues/88 and debian-devel discussion: http://lists.debian.org/20190716095726.GA18908@home.ouaza.com
- Fix #25 (closed): document nftables/nft
Get it reviewed
Ask Seth Arnold to review the AppArmor section (he offered us to do that, please put hertzog@debian.org in copy of your mail)
Fix the issues reported by the reviewer
Announce availability of updated chapter to volunteer reviewers