Add --nftset option, like --ipset but for the newer nftables.

Thanks to Chen Zhenge for the original patch, which I've
reworked. Any bugs down to SRK.

src/cache.c

@@ -2060,7 +2060,7 @@ void log_query(unsigned int flags, char *name, union all_addr *addr, char *arg,
     }
   else if (flags & F_IPSET)
     {
-      source = "ipset add";
+      source = type ? "ipset add" : "nftset add";
       dest = name;
       name = arg;
       verb = daemon->addrbuff;