net/tstun: move TUN failure diagnostics to OS-specific files

Mostly so the Linux one can use Linux-specific stuff in package
syscall and not use os/exec for uname for portability.

But also it helps deps a tiny bit on iOS.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tun_macos.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin && !ios
+// +build darwin,!ios
+
+package tstun
+
+import (
+	"os"
+
+	"tailscale.com/types/logger"
+)
+
+func init() {
+	tunDiagnoseFailure = diagnoseDarwinTUNFailure
+}
+
+func diagnoseDarwinTUNFailure(tunName string, logf logger.Logf) {
+	if os.Getuid() != 0 {
+		logf("failed to create TUN device as non-root user; use 'sudo tailscaled', or run under launchd with 'sudo tailscaled install-system-daemon'")
+	}
+	if tunName != "utun" {
+		logf("failed to create TUN device %q; try using tun device \"utun\" instead for automatic selection", tunName)
+	}
+}