INT 21 - European MSDOS 4.0 - SEND SIGNAL
        AH = 8Dh
        AL = signal number (see AH=8Ch)
        BH = signal argument
        BL = action
            00h send to entire command subtree
            01h send only to specified process
        DX = Process ID
Return: CF clear if successful
        CF set on error
            AX = error code (01h,06h)(see AH=59h)
Desc:   invoke the exceptional-condition handler for the specified process
Note:   error 06h may be returned if one or more of the affected processes
          have an error handler for the signal