A filter primitive that translates between strings and their
    corresponding external representations. Strings cannot be longer
    than the value specified with the maxsize parameter.

    While decoding, if *spp is NULL, this routine allocates the
    necessary storage to hold the NULL-terminated string and sets
    *spp to point to the allocated storage.

    This routine is the same as the xdr_wrapstring routine, except
    that this routine allows you to specify maxsize.