Add diagnostic entry point to authenticators

The authenticators should have a similar way to print diagnostic
information as filter and routers do. This allows the authenticators to
print the users in their own format.

In the future, all the diagnostic entry points should be changed so that
they return a structure that contains the information in a standard
form. This information can then be formatted in different ways by other
modules.

include/maxscale/service.h

@@ -258,6 +258,15 @@ int   service_refresh_users(SERVICE *service);
 /**
  * Diagnostics
  */
+
+/**
+ * @brief Print service authenticator diagnostics
+ *
+ * @param dcb     DCB to print to
+ * @param service The service to diagnose
+ */
+void service_print_users(DCB *, const SERVICE *);
+
 void       dprintAllServices(DCB *dcb);
 void       dprintService(DCB *dcb, SERVICE *service);
 void       dListServices(DCB *dcb);