This function, when called with extbuf as a NULL pointer and
    extlen as 0, returns the updated number of bytes in an extension
    header.
    If you specify extbuf as a valid pointer and valid extlen and
    align arguments, the function returns the same information as in
    the previous case, but also inserts the pad option, initializes
    the type and len fields, and returns a pointer to the location
    for the option content.
    After you call inet6_opt_append(), you can then use the data
    buffer directly or call inet6_opt_set_val() to specify the option
    contents.