src: use libnftables

src: use libnftables

This patch migrates nft to use the libnftables library, that is used
by the iptables over nftables compat utility as well. Most of the
conversion was pretty straight forward. Some small significant changes
happened in the handling of set element and immediate data abstraction
that libnl provides. libnftables is a bit more granular since it splits
the struct nfnl_nft_data into three attributes: verdict, chain and plain
data (used in maps).

I have added a new file src/mnl.c that contains the low level netlink
communication that now resides in nftables source tree instead of
the library. This should help to implement the batching support using
libmnl in follow up patches.

I also spent some significant amount of time running my tests to make
sure that we don't increase the number of bugs that we already have
(I plan to provide a list of those that I have detected and diagnosed,
so anyone else can help us to fix them).

As a side effect, this change should also prepare the ground for
JSON and XML support anytime soon.

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