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.

internal/router/ddwrt/ddwrt.go

@@ -57,12 +57,6 @@ func (d *Ddwrt) PreRun() error {
 	return ntp.Wait()
 }
 
-func (d *Ddwrt) Configure() error {
-	d.cfg.Listener["0"].IP = "127.0.0.1"
-	d.cfg.Listener["0"].Port = 5354
-	return nil
-}
-
 func (d *Ddwrt) Setup() error {
 	// Already setup.
 	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val == "1" {