cli: allow option groups to force async/sync operations

Some operations are clearly asynchronous (e.g. monitoring modem state), while
others are clearly synchronous (e.g. printing modem info). So just allow option
groups to force the operation to be async or sync based on the detected action.

cli/mmcli.h

@@ -24,7 +24,9 @@
 #define __MMCLI_H__
 
 /* Common */
-void          mmcli_async_operation_done (void);
+void          mmcli_async_operation_done  (void);
+void          mmcli_force_async_operation (void);
+void          mmcli_force_sync_operation  (void);
 
 /* Manager group */
 GOptionGroup *mmcli_manager_get_option_group (void);