- remove net.inet6.ip6.nd6_proxyall. introduce proxy NDP code works
just like "arp -s".
- revise source address selection.
be more careful about use of yet-to-be-valid addresses as source.
- as router, transmit ICMP6_DST_UNREACH_BEYONDSCOPE against out-of-scope
packet forwarding attempt.
- path MTU discovery takes care of routing header properly.
- be more strict about mbuf chain parsing.