INT 2F U - DOS 3+ internal - MAKE DISK BUFFER MOST-RECENTLY USED
        AX = 1207h
        DS:DI -> disk buffer
Return: nothing
Desc:   move the indicated buffer to the end of the disk buffer chain (least-
          recently used is first); under DOS 3.3, the buffer is then moved to
          the start of the disk buffer chain if it was marked unused
Note:   can be called only from within DOS