make "ip6fw zero" work.  FreeBSD PR 20522.  FreeBSD 1.5 -> 1.6.
make "ip6fw zero" work.  FreeBSD PR 20522.  FreeBSD 1.5 -> 1.6.

MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame