lib/pmp: Add NAT-PMP support (ref #698)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/2968

cmd/syncthing/main.go

@@ -45,9 +45,12 @@ import (
 	"github.com/syncthing/syncthing/lib/symlinks"
 	"github.com/syncthing/syncthing/lib/tlsutil"
 	"github.com/syncthing/syncthing/lib/upgrade"
-	_ "github.com/syncthing/syncthing/lib/upnp"
 	"github.com/syncthing/syncthing/lib/util"
 
+	// Registers NAT service providers
+	_ "github.com/syncthing/syncthing/lib/pmp"
+	_ "github.com/syncthing/syncthing/lib/upnp"
+
 	"github.com/thejerf/suture"
 )