dnsmasq: nftset: serve from ipset config

Use existing ipset configs as source for nftsets to be compatible with
existing configs. As the OS can either have iptables XOR nftables
support, it's fine to provide both to dnsmasq. dnsmasq will silently
fail for the present one. Depending on the dnsmasq compile time options,
the ipsets or nftsets option will not be added to the dnsmasq config
file.

dnsmasq will try to add the IP addresses to all sets, regardless of the
IP version defined for the set. Adding an IPv6 to an IPv4 set and vice
versa will silently fail.

Signed-off-by: Mathias Kresin <dev@kresin.me>