cmd/cli: ensure all ifaces operation is set correctly

Since ctrld process does not rely on the global variable iface anymore
during runtime, ctrld client's operations must be updated to reflect
this change, too.

cmd/cli/control_server.go

@@ -31,6 +31,11 @@ const (
 	sendLogsPath     = "/logs/send"
 )
 
+type ifaceResponse struct {
+	Name string `json:"name"`
+	All  bool   `json:"all"`
+}
+
 type controlServer struct {
 	server *http.Server
 	mux    *http.ServeMux
@@ -205,15 +210,20 @@ func (p *prog) registerControlServerHandler() {
 		w.WriteHeader(http.StatusBadRequest)
 	}))
 	p.cs.register(ifacePath, http.HandlerFunc(func(w http.ResponseWriter, request *http.Request) {
+		res := &ifaceResponse{Name: iface}
 		// p.setDNS is only called when running as a service
 		if !service.Interactive() {
 			<-p.csSetDnsDone
 			if p.csSetDnsOk {
-				w.Write([]byte(iface))
-				return
+				res.Name = p.runningIface
+				res.All = p.requiredMultiNICsConfig
 			}
 		}
-		w.WriteHeader(http.StatusBadRequest)
+		if err := json.NewEncoder(w).Encode(res); err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			http.Error(w, fmt.Sprintf("could not marshal iface data: %v", err), http.StatusInternalServerError)
+			return
+		}
 	}))
 	p.cs.register(viewLogsPath, http.HandlerFunc(func(w http.ResponseWriter, request *http.Request) {
 		lr, err := p.logReader()