MFC: We couldn't specify the rule for filtering tunnel traffic since an
IPv6 support was committed:
- Stop treating ip' and ipv6' as special in `proto' option as they conflict with /etc/protocols.
- Disuse ipv4' in proto' option as it is corresponding to `ipv6'.
- When protocol is specified as numeric, treat it as it is even it is 41 (ipv6).
- Allow zero for protocol as it is valid number of `ip'.
sbin/ipfw/ipfw.8: 1.180
src/sbin/ipfw/ipfw2.c: 1.80