DOS Execute command (Internal)

        ES:SI -> counted CR-terminated command string

Note:   The top-level command.com executes the command.
        All registers including SS and SP are destroyed as in INT 21/AH=4Bh.
        Since COMMAND.COM processes the string as if typed from the keyboard,
        the transient portion needs to be present, and the calling program
        must ensure that sufficient memory to load the transient portion can
        be allocated by DOS if necessary.