wgengine: start network logger in Userspace.Reconfig (#5908)

If the wgcfg.Config is specified with network logging arguments,
then Userspace.Reconfig starts up an asynchronous network logger,
which is shutdown either upon Userspace.Close or when Userspace.Reconfig
is called again without network logging or route arguments.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

wgengine/netlog/logger_test.go

@@ -59,7 +59,7 @@ func TestResourceCheck(t *testing.T) {
 	var l Logger
 	var d fakeDevice
 	for i := 0; i < 10; i++ {
-		must.Do(l.Startup(logtail.PrivateID{}, logtail.PrivateID{}, &d, &router.Config{}))
+		must.Do(l.Startup(logtail.PrivateID{}, logtail.PrivateID{}, &d))
 		l.ReconfigRoutes(&router.Config{})
 		must.Do(l.Shutdown(context.Background()))
 		c.Assert(d.toggled, qt.Equals, 2*(i+1))