MFen: Sync with revision 1.30 of the English article.

Improvements to the suggested firewall ruleset from 1.27.
Marking up of nmap as a port from 1.26.
Improvements to ppp_nat instructions from 1.25.
Delete ipfw rule numbers from 1.24.
Sysctl tip for ipfw log limit from 1.20
Relative links to FreeBSD docs from 1.19.