added stricter validation on source address (for reference purpose only)

(forced commit to update the commit log of rev 1.37)

the diff was taken from FreeBSD's rev 1.7.4.1.4.1 of ipcomp_input.c
(which was then incorporated to NetBSD, whose rev is 1.28).

merbed back a fix to invalid pointer access

(FreeBSD's rev 1.28 of ipcomp_input.c)

removed garbage

update comment on the use of strncpy()

make it compile on 4.1-current XXX quickhack

fix problems with the previous commit.

verified compilation on openbsd 4.1-current.

RFC3946 (by pekka savola) has all the details on 6to4 packet filtering.

refer draft-{itojun,cmetz}-*-harmful*.txt and
draft-ietf-v6ops-security-overview-06.txt (again, by pekka savola).

oops

removed duplicated macro definition.