macOS mDNSResponder permanently occupies UDP port 5350, which is the
NAT-PMP client port that webtorrent binds when initializing its NAT
traversal. The bind fails asynchronously with EADDRINUSE, and since
the PMP client is a raw EventEmitter with no error listener, it
surfaces as an uncaughtException that kills the app the moment a
download starts.
NAT-PMP can never succeed on macOS because the port is permanently
taken, so disable it on darwin only and let UPnP handle NAT traversal
instead. Other platforms are unaffected.
Closes#22
