Allows to create a new SEQ frame.
SEQ frame are especial to BEEP because they allows to control how flow control is handled based on channel window size.
|channel_num ||The channel number where the channel will|
|ackno ||The sequence number expected to be received at the time the SEQ frame was generated.|
|window_size ||The window size that the SEQ frame will acknowledge.|
- A newly allocated raw SEQ frame. Returned value should be deallocated using axl_free.