Signals the client that it should save a copy of its current state such that it could be restarted later in exactly the same state as it is at the time of signal emission.
If the state is simple enough to be encoded in the application's command line, set_restart_command can be used to set that command line. For more complex state data, get_state_file should be used.
The application should attempt to save its state as quickly as possible, and MUST NOT interact with the user as a part of saving state.