Eagerly check minimum packet lengths when matching packet protocols

* src/pf/expand.lua: Give names to protocol numbers.  If we match a
  packet for a given protocol type, eagerly assert that the packet is
  bigger than the minimum size.  This should result in some check
  elimination, as when accessing the src and dst addresses of ip
  packets.

* doc/host-127.0.0.1.md:
* doc/icmp-or-tcp-or-udp.md:
* doc/portrange-0-6000.md:
* doc/tcp-port-80.md: Regenerate.