wgengine/monitor: make the darwin link monitor work in the sandbox too

Previously tailscaled on macOS was running "/sbin/route monitor" as a
child process, but child processes aren't allowed in the Network
Extension / App Store sandbox. Instead, just do what "/sbin/route monitor"
itself does: unix.Socket(unix.AF_ROUTE, unix.SOCK_RAW, 0) and read that.

We also parse it now, but don't do anything with the parsed results yet.

We will over time, as we have with Linux netlink messages over time.

Currently any message is considered a signal to poll and see what changed.

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

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,!darwin android darwin,redo
+// +build !linux,!freebsd,!windows,!darwin android
 
 package monitor