Implement nftables support

Shorewall should provide support for nftables in addition to the current iptables support.

Tom offered the following suggestion via email:

You could rewrite Shorewall::Chains::format_rule() to generate nftables rules rather than iptables rules (that is similar to what the iptables compatibility stuff in Netfilter does). There's more to do, of course, but that would go a long way toward making Shorewall generate nftables. Looking for usage of '$tool' should find a large percentage of the remaining work.

Additional useful context might be found in this shorewall-users mailing list thread: https://sourceforge.net/p/shorewall/mailman/shorewall-users/thread/20190722103932.C10A26C803F9%40dd34104.kasserver.com/#msg36723248