Rework, changes:
- Add "Choosing a Firewall" comparison section.
- Add "Traffic Shaping with Dummynet" section, demote ALTQ to a legacy note
- Rewrite PF examples
- Add set skip on lo0, syncookies, ICMPv6/NDP guidance, etc
- Modernize interfaces, mail ports, etc
- Fix inbound ICMP/ICMPv6 to allow PMTUD and neighbor discovery
- Trim IPFILTER to pointer-level, compress FTP proxy, drop spamd/greylisting