MXS-862: Add authenticator options and instances

Authenticators now have a similar mechanism to the `router_options`
parameter which enables configurable authentication.

The authenticators also have a new initialize entry point which is similar
to the createInstance entry point of the filters and routers. The value of
`authenticator_options` is passed as a parameter to this function. The
return vaulue of the `initialize` entry point is passed to the `create`
entry point.

server/modules/authenticator/gssapi_auth.h

@@ -42,7 +42,7 @@ typedef struct gssapi_auth
 } gssapi_auth_t;
 
 /** These functions can used for the `create` and `destroy` entry points */
-void* gssapi_auth_alloc();
+void* gssapi_auth_alloc(void *instance);
 void gssapi_auth_free(void *data);
 
 /** Report GSSAPI errors */