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/authenticator.h

@@ -60,6 +60,11 @@ struct servlistener;
  *                      entry point.
  *
  *      loadusers       Load or update authenticator user data
+ *
+ *      diagnostic      Print diagnostic output to a DCB
+ *
+ *      reauthenticate  Reauthenticate a user
+ *
  * @endverbatim
  *
  * This forms the "module object" for authenticator modules within the gateway.
@@ -76,6 +81,7 @@ typedef struct mxs_authenticator
     void  (*free)(struct dcb *);
     void  (*destroy)(void *);
     int   (*loadusers)(struct servlistener *);
+    void  (*diagnostic)(struct dcb*, struct servlistener *);
 
     /** This entry point was added to avoid calling authenticator functions
      * directly when a COM_CHANGE_USER command is executed. */