INT 21 - DoubleDOS - SET/RESET KEYBOARD CONTROL FLAGS
        AH = E8h
        AL = 00h set flags for this program
           = 01h set flags for other program
        DX = keyboard control flags (see below)
Return: DX = previous flags
Notes:  disabling Ctrl-PrtSc will allow the program to intercept the keystroke;
          disabling any of the other keystrokes disables them completely
        identical to AH=F8h
SeeAlso: AH=E1h"DoubleDOS",AH=E2h"DoubleDOS",AH=E3h"DoubleDOS"
SeeAlso: AH=F8h"DoubleDOS"

Bitfields for keyboard control flags:
 bit 0  menu
 bit 1  exchange
 bit 2  entire keyboard enable/disable
 bit 3  Ctrl-C
 bit 4  Ctrl-PrtSc
 bit 5  Alt/Erase
 bit 6  Ctrl-Break
 bit 7  Ctrl-NumLock
 bit 8  shift-PrtSc
 bit 9-13 undefined
 bit 14 cancel key (clear keyboard buffer)
 bit 15 suspend key
Note:   setting a bit enables the corresponding key or operatin, clearing a
          bit disables it

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