INT 2F - DR DOS 6.0 TaskMAX - CONVERT TASK INDEX TO TASK ID                     
	AX = 270Ah
	DX = task index
Return: DX = task ID (FFFFh if index invalid)
Note:	task IDs stay constant, while indexes can change when other tasks are
	  deleted
SeeAlso: AX=2701h,AX=270Bh