feat(daemon): add ExternalMode reboot

[anpep]

This patch adds a new daemon.ExternalMode reboot mode that does nothing, so that code in charge of starting and stopping the Pebble daemon can run custom logic after the daemon has stopped.

When ExternalMode is set and the Pebble daemon stops after a system restart has been requested, daemon.Stop() returns the special daemon.ErrRestartExternal error that can be detected by the caller in order to execute custom reboot logic.

Furthermore, the fallback reboot handler is set to syscallModeReboot(), so that a system reboot is forced after rebootWaitTimeout is reached. In other words, the code responsible for handling the reboot with custom logic has a time span given by rebootWaitTimeout to perform a system reboot, or it will be forced.

func exampleUsage() error {
    /* ... */
    daemon.SetRebootMode(daemon.ExternalMode)
    d, err := daemon.New(...)
    if err != nil {
        return err
    }
    /* ... */

    sigs := make(chan os.Signal, 1)
    signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM, syscall.SIGQUIT)
    select {
    case <-sigs:
        // Exiting due to signal
    case <-d.Dying():
        // Exiting due to daemon error/requested reboot
    }

    err := d.Stop()
    if errors.Is(err, daemon.ErrRestartExternal) {
        // Custom reboot logic (~10min window before reboot(2) is called)
        /* ... */
        return nil
    }
    return err
}

[benhoyt]

This looks reasonable to me, thanks. (Though I had one clarifying question.)

The only other thing I'd suggest is to use errors.Is instead of err == daemon.ErrRestartExternal in your PR description (though I see you've used it in the test code).

[flotter]

Looks good for me now, thanks @anpep

[benhoyt]

Thanks!