Set escape routine address

        Entry:
          AH = 12h
          BH = 14h
          BL = message modifier
               00h handle is DWORD on top of stack
               01h define user stream
               04h intercept keystrokes from KEYBOARD to a window (handle on stack)
               05h intercept keystrokes from task's default KEYBOARD to a window
          STACK: (if window)   DWORD user stream number (14h-1Fh)
                               DWORD address of FAR user stream handler
                 (if keyboard) DWORD address of FAR filter function

        The keyboard filter function is called when the keyboard is in
        field mode.  On entry:

                AL = character
                AH = 00h or extended ASCII code if AL = 00h
                BL = field number
                CH = cursor column
                CL = cursor row
                DL = field type modifier (sixth item in field table entry)
                DH = ??? (seventh item in field table entry)
                ES:SI = window's handle

        (Also, in DV 2.00, DS:DI points to the field table entry.
         This may change in other versions)

        The filter function should return

                AH = 00h use keystroke
                     01h ignore keystroke
                     >1  beep and ignore keystroke