netfilter: conntrack: tag conntracks picked up in local out hook

This allows to identify flows that originate from local machine
in a followup patch.

It would be possible to make this a ->status bit instead.
For now I did not do that yet because I don't have a use-case for
exposing this info to userspace.

If one comes up the toggle can be replaced with a status bit.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>