Expects: AX    0019H
CX    event mask of desired handler (as used in INT 33H 0018H)
                                                                  
Returns: CX    event mask of handler or...
0000H = no handler matches
ES:DX address of matching event handler
                                                                  
Info: Use this function to obtain the address and mask of an alternate
event handler installed via INT 33H 0018H (which see for the bit
values for CX).

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INT 33H 0019H: Query User Alternate Event Handler