Currently the nat extension is always attached as soon as nat module is
loaded.  However, most NAT uses do not need the nat extension anymore.

Prepare to remove the add-nat-by-default by making those places that need
it attach it if its not present yet.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>