>>Synopsis:       Forwarding loop on down interfaces

	Followup to this PR (from nrt@iij.ad.jp - emailed privately in
	Japanese).

	The logic on the PR (which is already committed) does not work
	properly when:
	- same IPv4 address (X) is assigned to two or more interfaces
	- one of them is !IFF_UP
	In this case, packet with ip_dst = X can be mistakingly dropped.

	I really need clarification on what IFF_UP means, and the right
	behavior.  In the following, suppose we've got a packet with
	ip_dst = X.

	Traditional 4.4BSD (and open/freebsd):
		accept packets regardless form IFF_UP flag.
		I believe this is because of the following thinking:
		- IFF_UP means "layer-2 is down", not others
		- IPv4 address lookup in "goto ours" logic should not consult
		  layer-2 thing.  also, IPv4 address lookup is based on weak
		  model (we accept the packet, even if the incoming interface
		  != interface which has X)
	NetBSD 1.4.1:
		accept packets, only when interface which has X is IFF_UP.
		non-accepted packet can be forwarded till TTL goes to 0
		in the kernel (which is not pleasant)
	NetBSD-current (with nrt's patch):
		accept packets, only when interface which has X is IFF_UP.
		non-accepted packet result in icmp unreach.
		(there's some mistake in icmp unreach decision)

	We can fix it in multiple ways.  Which behavior is 'the right one'?

itojun