iptables: snat: add randomize-full support

This patch provides the userspace part for snat in order to make
randomize-full support available in {ip,nf}tables. It allows for
enabling full port randomization that was motivated in [1] and
introduced to the kernel in [2].

Joint work between Hannes Frederic Sowa and Daniel Borkmann.

 [1] https://sites.google.com/site/hayashulman/files/NIC-derandomisation.pdf
 [2] http://patchwork.ozlabs.org/patch/304306/

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>