miniupnpd: create iptables and nftables variant

miniupnpd: create iptables and nftables variant

The next OpenWrt stable release aims to use firewall4 by default. As
this uses nftables as backend, miniupnpd will no longer work. Create an
iptables and nftables variant of the miniupnpd package so that miniupnpd
can be used with either firewall variant.

See #16818 for more info.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>