SilcTask
    silc_schedule_task_add_fd(SilcSchedule schedule, SilcUInt32 fd,
                              SilcTaskCallback callback, void *context);

    Add file descriptor task to scheduler.  The `fd' may be either real
    file descriptor, socket or on some platforms an opaque file descriptor
    handle.  To receive events for the file descriptor set the correct
    request events with silc_schedule_set_listen_fd function.

    The task will be initially set for SILC_TASK_READ events.  Setting that
    event immediately after this call returns is not necessary.

    This returns the new task or NULL on error.  If a task with `fd' has
    already been added this will return the existing task pointer.