src: add `flush ruleset'

src: add `flush ruleset'

This patch adds the `flush ruleset' operation to nft.

The syntax is:
 % nft flush ruleset [family]

To flush all the ruleset (all families):
 % nft flush ruleset

To flush the ruleset of a given family:
 % nft flush ruleset ip
 % nft flush ruleset inet

This flush is a shortcut operation which deletes all rules, sets, tables
and chains.
It's possible since the modifications in the kernel to the NFT_MSG_DELTABLE
API call.

Users can benefit of this operation when doing an atomic replacement of the
entire ruleset, loading a file like this:

 =========
 flush ruleset
 table ip filter {
 	chain input {
		counter accept
	}
 }
 =========

Also, users who want to simply clean the ruleset for whatever reason can do it now
without having to iterate families/tables.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>