pfil information, instead, struct protosw now contains a structure
which caontains list heads, etc. The per-protosw pfil struct is passed
to pfil_hook_get(), along with an in/out flag to get the head of the
relevant filter list. This has been done for only IPv4 and IPv6, at
present, with these patches only enabling filtering for IPPROTO_IP and
IPPROTO_IPV6, although it is possible to have tcp/udp, etc, dedicated
filters now also. The ipfilter code has been updated to only filter
IPv4 packets - next major release of ipfilter is required for ipv6.
change pfil_add_hook to put output filters at the tail of the queue,
while continuing to place input filters at the head of the queue. update
the two users of these functions, and document these changes.
fixes PR#4593.
