A found tunnel carries its MTU to the mesh

The host parses MTU from the found [Interface] and reports it, so the
mesh's interface can come up with the same MTU when it takes the tunnel
over. A path tuned to 1380 regresses to the 1420 default otherwise —
invisible to ping, fatal to TLS handshakes and transfers over that path
(novox/hq: the mesh had no MTU concept). Zero when the config named
none, and the mesh writes no MTU line then.

internal/tunnel/tunnel_test.go

@@ -170,3 +170,27 @@ func TestNoneUpIsAnOrdinaryAnswerAndSeveralIsAQuestion(t *testing.T) {
 		t.Errorf("naming a tunnel that is not up was not refused: %v", err)
 	}
 }
+
+func TestAFoundTunnelReadsItsMTU(t *testing.T) {
+	// A tuned path sets MTU in [Interface]; the mesh must carry it or the tunnel regresses to the
+	// default silently (novox/hq: a taken tunnel carries its MTU).
+	private, public := aKey(t)
+	withMTU := "# tuned\n[Interface]\nPrivateKey = " + private +
+		"\nListenPort = 51820\nAddress = 10.10.0.3/24\nMTU = 1380\n" +
+		"[Peer]\nPublicKey = " + public + "\nAllowedIPs = 10.10.0.1/32\n"
+	f, err := Parse([]byte(withMTU))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if f.MTU != 1380 {
+		t.Fatalf("MTU 1380 was not read; got %d", f.MTU)
+	}
+	// And a config with none leaves MTU zero, so the mesh writes no MTU line.
+	f2, err := Parse([]byte(aConfig(private, public)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if f2.MTU != 0 {
+		t.Fatalf("a config with no MTU must leave it zero; got %d", f2.MTU)
+	}
+}