[SECMARK]: Add xtables SECMARK target

Add a SECMARK target to xtables, allowing the admin to apply security
marks to packets via both iptables and ip6tables.

The target currently handles SELinux security marking, but can be
extended for other purposes as needed.

Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>