INT 33 - MS MOUSE v6.0+ - SET ALTERNATE MOUSE USER HANDLER
        AX = 0018h
        CX = call mask (see below)
        ES:DX -> FAR routine to be invoked on mouse events (see below)
Return: AX = status
            0018h if successful
            FFFFh on error
Notes:  up to three handlers can be defined by separate calls to this function,
          each with a different combination of shift states in the call mask;
          calling this function again with a call mask of 0000h undefines the
          specified handler (official documentation); specifying the same
          call mask and an address of 0000h:0000h undefines the handler (real
          life)
        some versions of the documentation erroneously reverse the order of
          the bits in the call mask
SeeAlso: AX=000Ch,AX=0014h,AX=0019h

Bitfields for 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
Note:   at least one of bits 5-7 must be set

User handler called with:
        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
Return: registers preserved
Note:   in text modes, the row and column will be reported as a multiple of
          the cell size, typically 8x8 pixels

.