iptables: extensions: libxt_ecn: Add translation to nft

iptables: extensions: libxt_ecn: Add translation to nft

Add translation of the ecn match to nftables.

Examples:
  # iptables-translate -A INPUT -m ecn --ecn-ip-ect 0
  nft add rule ip filter INPUT ip ecn not-ect counter

  # iptables-translate -A INPUT -m ecn --ecn-ip-ect 1
  nft add rule ip filter INPUT ip ecn ect1 counter

  # iptables-translate -A INPUT -m ecn --ecn-ip-ect 2
  nft add rule ip filter INPUT ip ecn ect0 counter

  # iptables-translate -A INPUT -m ecn --ecn-ip-ect 3
  nft add rule ip filter INPUT ip ecn ce counter

  # iptables-translate -A INPUT -m ecn ! --ecn-ip-ect 0
  nft add rule ip filter INPUT ip ecn != not-ect counter

  # iptables-translate -A INPUT -m ecn ! --ecn-ip-ect 1
  nft add rule ip filter INPUT ip ecn != ect1 counter

  # iptables-translate -A INPUT -m ecn ! --ecn-ip-ect 2
  nft add rule ip filter INPUT ip ecn != ect0 counter

  # iptables-translate -A INPUT -m ecn ! --ecn-ip-ect 3
  nft add rule ip filter INPUT ip ecn != ce counter

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