shadowsocks-libev: convert to using nft

shadowsocks-libev: convert to using nft

It will be mostly implemented with ucode templates installed at
/usr/share/ss-rules and called from init script.  The generated nftables
rules will be stored at /etc/nftables.d/

Incompatible changes were introduced as described in the README.md file

 - Netfilter ipset was replaced with nftables sets
 - UCI options ipt_args and dst_forward_recentrst of section ss_rules
   are now deprecated.  The former does not apply to nftables.  The
   later not yet implemented with nftables.

Signed-off-by: Yousong Zhou <redacted>