This routine allows an application to verify the existence
    of, and return components of, a file specification. Wildcard
    operations are permitted. The routine uses the $PARSE and $SEARCH
    system services to seek the file specification.

    If no wildcards are included in the file specification string
    and the result-string returns a zero (0) length string, no
    file was found. If wildcard characters were present in the file
    specification and the result-string returns a zero (0) length
    string, there are no more files that match the wildcards.

    To find all the files that match a wildcard specification,
    repeatedly call this routine, passing the same arguments, until
    the routine returns a zero-length result string.

    The TPU$FILE_SEARCH routine is called by DECTPU when the TPU
    built-in procedure FILE_SEARCH is executed from TPU code. The
    return value of the built-in procedure is the string returned in
    the result-string argument.