iptables: default to ip(6)tables-nft

iptables: default to ip(6)tables-nft

OpenWrt now uses firewall4 (nft) by default,
so iptables should also default to nftables backend.

When multiple packages provide the same virtual package,
opkg pick the first one by alphabetical order,
so we rename iptables-legacy to iptables-zz-legacy and add
iptables-legacy in PROVIDES.

We also need to remove IPTABLES_NFTABLES config as
this cause recursive dependencies.

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
(cherry picked from commit 35fec487e30f05c81bd135326a993dad7f861812)