envknob,net/tstun,wgengine: use TS_DEBUG_MTU consistently

Noted on #5915 TS_DEBUG_MTU was not used consistently everywhere.
Extract the default into a function that can apply this centrally and
use it everywhere.

Added envknob.Lookup{Int,Uint}Sized to make it easier to keep CodeQL
happy when using converted values.

Updates #5915

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

net/tstun/mtu_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+package tstun
+
+import (
+	"os"
+	"testing"
+)
+
+func TestDefaultMTU(t *testing.T) {
+	orig := os.Getenv("TS_DEBUG_MTU")
+	defer os.Setenv("TS_DEBUG_MTU", orig)
+
+	os.Setenv("TS_DEBUG_MTU", "")
+	if DefaultMTU() != 1280 {
+		t.Errorf("DefaultMTU() = %d, want 1280", DefaultMTU())
+	}
+
+	os.Setenv("TS_DEBUG_MTU", "9000")
+	if DefaultMTU() != 9000 {
+		t.Errorf("DefaultMTU() = %d, want 9000", DefaultMTU())
+	}
+
+	os.Setenv("TS_DEBUG_MTU", "123456789")
+	if DefaultMTU() != maxMTU {
+		t.Errorf("DefaultMTU() = %d, want %d", DefaultMTU(), maxMTU)
+	}
+}