Int 2F Fn 270A  - Dr Dos 6.0 Taskmax - Convert Task Index To Task Id       [T]

   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

See Also: AX=2701h,AX=270Bh