INT 16 - PC Tools v5.1+ DESKTOP API - DISPLAY HELP LINE
        AX = FFF2h
        DS:SI -> ASCIZ function key label string (each label preceded by '[')
                or help text
Return: AX destroyed
Notes:  available only when popped up
        if the specified string does not start with '[', it is displayed
          centered on the bottom line, else the function key labels are shown