wgengine/monitor: add a darwin implementation for tailscaled mode

Tangentially related to #987, #177, #594, #925, #505

Motivated by rebooting a launchd-controlled tailscaled and it going
into SetNetworkUp(false) mode immediately because there really is no
network up at system boot, but then it got stuck in that paused state
forever, without a monitor implementation.

wgengine/monitor/monitor_unsupported.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux,!freebsd,!windows android
+// +build !linux,!freebsd,!windows,!darwin android darwin,redo
 
 package monitor