freetdm: - Added ftdm_usage command to check if a channel has calls (ie, is busy)

- Refactored ftdm CLI management to allow standalone APIs to be registered
         - Minor logging changes here and there

libs/freetdm/src/ftdm_io.c

@@ -5116,6 +5116,7 @@ static ftdm_status_t load_config(void)
 	sprintf(chan_config.group_name, "__default");
 
 	if (!ftdm_config_open_file(&cfg, cfg_name)) {
+		ftdm_log(FTDM_LOG_ERROR, "Failed to open configuration file %s\n", cfg_name);
 		return FTDM_FAIL;
 	}