INT 31 P - DPMI 0.9+ - SET DESCRIPTOR
        AX = 000Ch
        BX = LDT selector
        ES:(E)DI -> 8-byte buffer containing descriptor
Return: CF clear if successful
        CF set on error
            AX = error code (DPMI 1.0+) (8021h,8022h,8025h) (see AX=0000h)
Notes:  16-bit programs use ES:DI as pointer, 32-bit must use ES:EDI
        only modify descriptors allocated with INT 31/AX=0000h
        DPMI 1.0+ automatically reloads any segment registers containing the
          selector being modified
        not supported by MS Windows 3.0 in Standard mode