Split ip_ecn code into its own module, so it can be shared between
gif(4), stf(4), and ipsec(4). Without this, loading the if_gif
module can result in redefined global symbols if either ipsec(4) or
stf(4) but not gif(4) is built into the kernel.

Fixes PR kern/52795 (as reported by martin@ via irc).

XXX pullup to netbsd-8

branches: 1.16.10;
sprinkle _KERNEL_OPT

branches: 1.15.102; 1.15.122;
Import of TCP ECN algorithm for congestion control.
Both available for IPv4 and IPv6.
Basic implementation test results are available at
http://netbsd-soc.sourceforge.net/projects/ecn/testresults.html.

Work sponsored by the Google Summer of Code project 2006.
Special thanks to Kentaro Kurahone, Allen Briggs and Matt Thomas for their
help, comments and support during the project.

branches: 1.14.4; 1.14.8;
merge ktrace-lwp.

branches: 1.13.6;
ANSIfy function prototypes. (Still have about 3/5ths of the C files in
netinet to go...)

branches: 1.12.10; 1.12.18; 1.12.20;
correct variable initialization. reported by fujitsu folks

add RCSIDs

branches: 1.10.2;
correct ecn consideration on tunnel encap/decap. sync with kame.

branches: 1.9.2;
fix compilation without INET. fix confusion between ipsecstat and ipsec6stat.
sync with kame.

remove too much portability code in KAME, to improve readability.

sync with latest KAME (rcsid only).

branches: 1.6.2; 1.6.8;
sync with recent KAME.
- loosen ipsec restriction on packet diredtion.
- revise icmp6 redirect handling on IsRouter bit.
- tcp/udp notification processing (link-local address case)
- cosmetic fixes (better code share across *BSD).

remove reference to in6_systm.h (file itself will be removed afterwords)

sync with KAME/NetBSD 1.4, SNAP kit 19990705.
key changes are:
- icmp6 redirect fix (dst check)
- revised ip6 multicast check for loopback i/f
- several RCS ID cleanups

RCS ID police.

branches: 1.2.2;
IPv6 kernel code, based on KAME/NetBSD 1.4, SNAP kit 19990628.
(Sorry for a big commit, I can't separate this into several pieces...)
Pls check sys/netinet6/TODO and sys/netinet6/IMPLEMENTATION for details.

- sys/kern: do not assume single mbuf, accept chained mbuf on passing
data from userland to kernel (or other way round).
- "midway" ATM card: ATM PVC pseudo device support, like those done in ALTQ
package (ftp://ftp.csl.sony.co.jp/pub/kjc/).
- sys/netinet/tcp*: IPv4/v6 dual stack tcp support.
- sys/netinet/{ip6,icmp6}.h, sys/net/pfkeyv2.h: IETF document assumes those
file to be there so we patch it up.
- sys/netinet: IPsec additions are here and there.
- sys/netinet6/*: most of IPv6 code sits here.
- sys/netkey: IPsec key management code
- dev/pci/pcidevs: regen

In my understanding no code here is subject to export control so it
should be safe.

branches: 1.1.2;
file ip_ecn.c was initially added on branch kame.