netfilter: conntrack: add nf_conntrack_events autodetect mode

This adds the new nf_conntrack_events=2 mode and makes it the
default.

This leverages the earlier flag in struct net to allow to avoid
the event extension as long as no event listener is active in
the namespace.

This avoids, for most cases, allocation of ct->ext area.
A followup patch will take further advantage of this by avoiding
calls down into the event framework if the extension isn't present.

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