[SECMARK]: Add CONNSECMARK xtables target

Add a new xtables target, CONNSECMARK, which is used to specify rules
for copying security marks from packets to connections, and for
copyying security marks back from connections to packets.  This is
similar to the CONNMARK target, but is more limited in scope in that
it only allows copying of security marks to and from packets, as this
is all it needs to do.

A typical scenario would be to apply a security mark to a 'new' packet
with SECMARK, then copy that to its conntrack via CONNMARK, and then
restore the security mark from the connection to established and
related packets on that connection.

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>