all: rework fetching/generating config in cd mode

Config fetching/generating in cd mode is currently weird, error prone,
and easy for user to break ctrld when using custom config.

This commit reworks the flow:

 - Fetching config from Control D API.
 - No custom config, use the current default config.
 - If custom config presents, but there's no listener, use 0.0.0.0:53.
 - Try listening on current ip+port config, if ok, ctrld could be a
   direct listener with current setup, moving on.
 - If failed, trying 127.0.0.1:53.
 - If failed, trying current ip + port 5354
 - If still failed, pick a random ip:port pair, retry until listening ok.

With this flow, thing is more predictable/stable, and help removing the
Config interface for router.

cmd/ctrld/main.go

@@ -30,7 +30,6 @@ var (
 	cdDev             bool
 	iface             string
 	ifaceStartStop    string
-	setupRouter       bool
 
 	mainLog       = zerolog.New(io.Discard)
 	consoleWriter zerolog.ConsoleWriter
@@ -39,7 +38,6 @@ var (
 func main() {
 	ctrld.InitConfig(v, "ctrld")
 	initCLI()
-	initRouterCLI()
 	if err := rootCmd.Execute(); err != nil {
 		mainLog.Error().Msg(err.Error())
 		os.Exit(1)