netfilter: introduce xt_id match

This commit implements a new netfilter match "xt_id" which can be used to
attach unsigned 32bit IDs to iptables rules.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

Backport of r41945

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42007 3c298f89-4303-0410-b956-a3cf2f4a3e73