Default to false.
We never set these "default default" values in any build, so we can simplify the configuration.
Note that we could simplify it even further by merging `USE_[UPNP,NATPMP]` and `ENABLE_[UPNP,NATPMP]`, which are now aliases of each-other, but all of these options are going away soon so it's not worth the extra effort.
This is a backport of core#26896 and core#28874