wgengine: remove Config.TUNName, require caller to create device.

Also factors out device creation and associated OS workarounds to
net/tun.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/tun/ifstatus_noop.go

@@ -0,0 +1,19 @@
+// 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.
+
+// +build !windows
+
+package tun
+
+import (
+	"time"
+
+	"github.com/tailscale/wireguard-go/tun"
+	"tailscale.com/types/logger"
+)
+
+// Dummy implementation that does nothing.
+func waitInterfaceUp(iface tun.Device, timeout time.Duration, logf logger.Logf) error {
+	return nil
+}