netfilter: nfnetlink_queue: add security context information

This patch adds an additional attribute when sending
packet information via netlink in netfilter_queue module.
It will send additional security context data, so that
userspace applications can verify this context against
their own security databases.

Signed-off-by: Roman Kubiak <r.kubiak@samsung.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>