src: add security context information

src: add security context information

This commit adds security context information structures
and functions.

This will allow userspace to find the security context of each
packet (if it exists) and make decisions based on that.
It should work for SELinux and SMACK.

Signed-off-by: Roman Kubiak <r.kubiak@samsung.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>