INT 21 - European MSDOS 4.0 - "AEXEC" - EXECUTE PROGRAM IN BACKGROUND
        AH = 80h
        CX = mode
            0000h place child in zombie mode on exit to preserve exit code
            0001h discard child process and exit code on termination
        DS:DX -> ASCIZ full program name
        ES:BX -> parameter block (as for AX=4B00h)
Return: CF clear if successful
            AX = Command Subgroup ID (CSID)
        CF set on error
            AX = error code (see AH=59h)
Desc:   asynchronously execute a program, creating a new process for it
Notes:  this function is called by the DETACH command
        there is a system-wide limit of 32 processes
        the CSID is used to identify all processes that have been spawned by
          a given process, whether directly or indirectly
        programs to be run in the background must use the new executable format
          (see AH=4Bh)
        background processes may only perform asynchronous (background) EXECs,
          either this function or AX=4B04h
        background processes may execute INT 11, INT 12, INT 21, INT 2A, and
          INT 2F at any time; they may execute INT 10 and INT 16 only while
          they have opened a popup screen via INT 2F/AX=1401h; no other
          interrupts may be executed from the background
        background processes may not use drive B: or overlay their code
          segments
SeeAlso: AX=8700h,INT 2F/AX=1400h"POPUP"