Format of Flow control subvector:
Offset	Size	Description	)
 00h	BYTE	length (02h-04h)
 01h	BYTE	19h (subvector "Flow control")
 02h	BYTE	XOFF character
 03h	BYTE	XON character
Note:	if length is 02h, flow control is disabled; if length is 03h, any
	  character will be accepted as XON after an XOFF
SeeAlso: #00313