wgengine: move link monitor to be owned by the engine, not the router

And make the monitor package portable with no-op implementations on
unsupported operating systems.

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

wgengine/monitor/monitor_unsupported.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2020 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 !linux,!freebsd
+
+package monitor
+
+func newOSMon() (osMon, error) { return nil, nil }