This function adds an IPv6 address to the end of the routing
    header under construction. The address pointed to by addr cannot
    be an IPv6 V4-mapped address or an IPv6 multicast address.
    The function increments the ip6r0_segleft member in the ip6_
    rthdr0 structure. The ip6_rthdr0 structure is defined in the
    IP6.H header file.
    Only routing header type 0 is supported.