This function initializes a buffer and buffer data for an IPv6
    routing header. The function sets the ip6r0_segleft, ip6r0_nxt,
    and ip6r0_reserved members in the ip6_rthdr0 structure to zero.
    In addition, it sets the ip6r0_type member to type and sets the
    ip6r0_len member based on the segments argument. The ip6_rthdr0
    structure is defined in the IP6.H header file.
    The application must allocate the buffer. Use the inet6_rth_
    space() function to determine the buffer size.
    Use the returned pointer as the first argument to the inet6_rth_
    add() function.