NAT simplifications

Hi all,

I've been digging through my patch collection, and there are some
significant NAT ones which I've updated. Mainly they simplify the NAT code, which I think is important since this area has been something of a quagmire (although I was delighted to find Krisztian Kovacs is brave enough to bugfind here).

Thoughts and test results welcome...
Rusty.

A bad analogy is like a leaky screwdriver -- Richard Braakman