This function reads an IPv6 routing header and writes a new
    routing header, reversing the order of addresses in the new
    header. The in and out parameters can point to the same buffer.
    The function sets the ip6r0_segleft member in the ip6_rthdr0
    structure to the number of segments (addresses) in the new
    header.
    The ip6_rthdr0 structure is defined in the IP6.H header file.