cmd/cli: implement auto captive portal detection

ControlD have global list of known captive portals that user can augment
with proper setup. However, this requires manual actions, and involving
restart ctrld for taking effects.

By allowing ctrld "leaks" DNS queries to OS resolver, this process
becomes automatically, the captive portal could intercept these queries,
and as long as it was passed, ctrld will resume normal operation.

cmd/cli/prog.go

@@ -105,6 +105,10 @@ type prog struct {
 	loopMu sync.Mutex
 	loop   map[string]bool
 
+	captivePortalMu          sync.Mutex
+	captivePortalCheckWasRun bool
+	captivePortalDetected    atomic.Bool
+
 	started       chan struct{}
 	onStartedDone chan struct{}
 	onStarted     []func()
@@ -240,6 +244,8 @@ func (p *prog) postRun() {
 		ns := ctrld.InitializeOsResolver()
 		mainLog.Load().Debug().Msgf("initialized OS resolver with nameservers: %v", ns)
 		p.setDNS()
+		p.csSetDnsDone <- struct{}{}
+		close(p.csSetDnsDone)
 	}
 }
 
@@ -534,8 +540,6 @@ func (p *prog) setDNS() {
 	setDnsOK := false
 	defer func() {
 		p.csSetDnsOk = setDnsOK
-		p.csSetDnsDone <- struct{}{}
-		close(p.csSetDnsDone)
 	}()
 
 	if cfg.Listener == nil {