This seems to break upnp for IPv4 in other ways, and we have since disabled IPv6 in the daemon while compiling anyhow. Revert "In order for miniupnpd to latch onto IPv6, we just need to give the interface name here, not the subnet. The daemon is smart enough to latch onto the right subnet(s) here anyhow."

This reverts commit bc8a1938f5c1797213bf849f2bdb88bca72d5089.