Move service user injection into MySQL authenticator

The MySQL authenticator now injects the service user into the list of
allowed users if loading of database users fails. This allows the removal
of common code in the binlogrouter and maxinfo modules.

server/modules/routing/binlog/blr.c

@@ -1790,7 +1790,7 @@ static void rses_end_locked_router_action(ROUTER_SLAVE *rses)
 
 static uint64_t getCapabilities()
 {
-    return (RCAP_TYPE_NO_RSESSION | RCAP_TYPE_NO_USERS_INIT);
+    return RCAP_TYPE_NO_RSESSION;
 }
 
 /**