SilcTask
    silc_schedule_task_add_signal(SilcSchedule schedule, int signal,
                                  SilcTaskCallback callback, void *context);

    Add platform specific process signal handler to scheduler.  On Unix
    systems the `signal' is one of the signal specified in signal(7).  On
    other platforms this function may not be available at all, and has no
    effect when called.  The event delivered to the `callback' is
    SILC_TASK_INTERRUPT.

    One signal may be registered only one callback.  Adding second callback
    for signal that already has one will fail.

    This function always returns NULL.  To remove signal from scheduler by
    the signal call silc_schedule_task_del_by_fd.