Add authentication plugin name to authenticator API

The authenticators can now declare the authentication plugin name. Right
now this is only relevant for MySQL authentication but for example the
HTTP module could implement both Basic and Digest authentication.

server/include/gw_authenticator.h

@@ -47,6 +47,7 @@ struct servlistener;
  *      authenticate    Carry out the authentication
  *      free            Free extracted data
  *      loadusers       Load or update authenticator user data
+ *      plugin_name     The protocol specific name of the authentication plugin.
  * @endverbatim
  *
  * This forms the "module object" for authenticator modules within the gateway.
@@ -60,6 +61,7 @@ typedef struct gw_authenticator
     int (*authenticate)(struct dcb *);
     void (*free)(struct dcb *);
     int (*loadusers)(struct servlistener *);
+    const char* plugin_name;
 } GWAUTHENTICATOR;
 
 /** Return values for the loadusers entry point */