basic nftables support

This is not perfect yet but the new -n flag trigger a switch from
ipset to nftables.

For example, if dom is started with:

 ./dom -f /var/log/suricata/eve.json -n nat -s libssh -vvv -i -m OpenSSH

it will add the IP to a set named 'libssh' which exists in the table
'nat'.

A working 'nat' table could then looks like:

 table ip nat {
   set libssh {
     type ipv4_addr
   }

   chain prerouting {
     type nat hook prerouting priority -150;
     ip saddr @libssh ip protocol tcp counter dnat 192.168.0.1:2200
   }
 }