INT 21 - DOS 5.0 - SET EXECUTION STATE
        AX = 4B05h
        DS:DX -> execution state structure (see below)
Return: CF clear if successful
            AX = 0000h
        CF set on error
            AX = error code (see AH=59h)
Note:   used by programs which intercept AX=4B00h to prepare new programs for
          execution (including setting the DOS version number).  No DOS, BIOS
          or other software interrupt may be called after return from this call
          before commencement of the child process.  If DOS is running in the
          HMA, A20 is turned off on return from this call.

Format of execution state structure:
Offset  Size    Description
 00h    WORD    reserved (00h)
 02h    WORD    type flags
                bit 0: program is an .EXE
                    1: program is an overlay
 04h    DWORD   pointer to ASCIZ name of program file
 08h    WORD    PSP segment of new program
 0Ah    DWORD   starting CS:IP of new program
 0Eh    DWORD   program size including PSP