extensions: libip[6]t_SNAT/DNAT: use the new nft syntax when do xlate

extensions: libip[6]t_SNAT/DNAT: use the new nft syntax when do xlate

After commit "src: add 'to' for snat and dnat" in nftables tree,
we should recommend the end user to use the new syntax.

Before this patch:
  # iptables-translate -t nat -A POSTROUTING -j SNAT --to-source 1.1.1.1
  nft add rule ip nat POSTROUTING counter snat 1.1.1.1
  # ip6tables-translate -t nat -A PREROUTING -j DNAT --to-destination
  2001::1
  nft add rule ip6 nat PREROUTING counter dnat 2001::1

Apply this patch:
  # iptables-translate -t nat -A POSTROUTING -j SNAT --to-source 1.1.1.1
  nft add rule ip nat POSTROUTING counter snat to 1.1.1.1
  # ip6tables-translate -t nat -A PREROUTING -j DNAT --to-destination
  2001::1
  nft add rule ip6 nat PREROUTING counter dnat to 2001::1

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>