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.

testhelper/config.go

@@ -19,6 +19,10 @@ func SampleConfig(t *testing.T) *ctrld.Config {
 	return &cfg
 }
 
+func SampleConfigStr(t *testing.T) string {
+	return sampleConfigContent
+}
+
 var sampleConfigContent = `
 [service]
 log_level = "info"