MXS-1929: Allow startup with no services

MaxScale can now be started with an empty configuration file and services
can be created at runtime. Filters cannot yet be created at runtime so
complete runtime creation of configurations is not yet possible.

server/core/internal/service.h

@@ -87,7 +87,7 @@ void service_destroy_instances(void);
  * Initialize and start all services. This should only be called once by the
  * main initialization code.
  *
- * @return Number of successfully started services
+ * @return Number of successfully started services or -1 on error
  */
 int service_launch_all(void);