netfilter: nf_tables: add SECMARK support

Add the ability to set the security context of packets within the nf_tables framework.
Add a nft_object for holding security contexts in the kernel and manipulating packets on the wire.

Convert the security context strings at rule addition time to security identifiers.
This is the same behavior like in xt_SECMARK and offers better performance than computing it per packet.

Set the maximum security context length to 256.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>