* many platforms:   be paranoid about malicious use of v4 mapped addr on v6 packet.   malicious party may try to use v4 mapped addr as source/dest to   confuse tcp/udp layer, or to bypass security checks,   for example, naive stack can mistakingly think a packet with   src = ::ffff:127.0.0.1 is from local node.   v4 mapped address is supposed to be for node local use, and should never   appear on the wire.
* many platforms:
  be paranoid about malicious use of v4 mapped addr on v6 packet.
  malicious party may try to use v4 mapped addr as source/dest to
  confuse tcp/udp layer, or to bypass security checks,
  for example, naive stack can mistakingly think a packet with
  src = ::ffff:127.0.0.1 is from local node.
  v4 mapped address is supposed to be for node local use, and should never
  appear on the wire.

  changed:
  tcp/udp/rawip layer for bsdi4, netbsd, freebsd2, bsdi3
  ip6 layer (ip6 input and routing header processing) for all OS.
  The latter change can be controversial.

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