Updated NAT fixes

[Steven Michaud]

> If you want patches evaluated for inclusion, please open
> change-request bug reports (or sw-bug bug reports for actual bugs)
> rather than just sending the patches here.

Done.

> I think that rather than applying your patches, I'll simply disable
> channel bindings completely in the ftpd.  This is consistent with what
> ssh and  SASL do.

That's fine by me.  It certainly is simpler than either of my
solutions.  And I agree with you that address checking doesn't buy you
much.  I've felt that all along, but I guessed that any NAT fixes
(mine or someone else's) would be more likely to be accepted if they
made as little change from the previous state of things as possible.

Turns out I was preaching to the converted :-)

And thanks, by the way, for the detailed and informative response.