cmd/cli: ensure goroutines that check DNS terminated

So changes to DNS after ctrld stopped won't be reverted by the goroutine
itself. The problem happens rarely on darwin, because networksetup
command won't propagate config to /etc/resolv.conf if there is no
changes between multiple running.

cmd/cli/cli.go

@@ -1333,6 +1333,10 @@ func run(appCallback *AppCallback, stopCh chan struct{}) {
 
 	close(waitCh)
 	<-stopCh
+
+	// Wait goroutines which watches/manipulates DNS settings terminated,
+	// ensuring that changes to DNS since here won't be reverted.
+	p.dnsWg.Wait()
 	for _, f := range p.onStopped {
 		f()
 	}