INT 33 - MS MOUSE - SET ALTERNATE MOUSE USER HANDLER
        AX = 0018h
        CX = call mask
             bit 0 call if mouse moves
             bit 1 call if left button pressed
             bit 2 call if left button released
             bit 3 call if right button pressed
             bit 4 call if right button released
             bit 5 call if shift button pressed during event
             bit 6 call if ctrl key pressed during event
             bit 7 call if alt key pressed during event
        ES:DX -> FAR routine
Return: AX = 0018h if successful
           = FFFFh on error
Notes:  when the subroutine is called, it is passed the following values:
          AX = condition mask (same bit assignments as call mask)
          BX = button state
          CX = cursor column
          DX = cursor row
          SI = horizontal mickey count
          DI = vertical mickey count
        up to three handlers can be defined by separate calls to this function
SeeAlso: AX=0014h,AX=0019h