cmd/xdpderper: add autodetection for default interface name

This makes deployment easier in hetrogenous environments.

Updates ENG-4274

Signed-off-by: James Tucker <james@tailscale.com>

net/netutil/default_interface_portable_test.go

@@ -0,0 +1,25 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package netutil
+
+import (
+	"testing"
+)
+
+func TestDefaultInterfacePortable(t *testing.T) {
+	ifName, addr, err := DefaultInterfacePortable()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Logf("Default interface: %s", ifName)
+	t.Logf("Default address: %s", addr)
+
+	if ifName == "" {
+		t.Fatal("Default interface name is empty")
+	}
+	if !addr.IsValid() {
+		t.Fatal("Default address is invalid")
+	}
+}