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

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

vendor/github.com/jackpal/gateway/gateway_test.go

@@ -0,0 +1,10 @@
+package gateway
+
+import "testing"
+
+func TestGateway(t *testing.T) {
+	ip, err := DiscoverGateway()
+	if err != nil {
+		t.Errorf("DiscoverGateway() = %v,%v", ip, err)
+	}
+}