add ipset to nftables translation infrastructure

add ipset to nftables translation infrastructure

This patch provides the ipset-translate utility which allows you to
translate your existing ipset file to nftables.

The ipset-translate utility is actually a symlink to ipset, which checks
for 'argv[0] == ipset-translate' to exercise the translation path.

You can translate your ipset file through:

	ipset-translate restore < sets.ipt

This patch reuses the existing parser and API to represent the sets and
the elements.

There is a new ipset_xlate_set dummy object that allows to store a
created set to fetch the type without interactions with the kernel.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>