ip6_pcbopt
error = ip6_pcbopt(IPV6_HOPLIMIT,
error = ip6_pcbopt(optname, (u_char *)&optval,
error = ip6_pcbopt(optname, optbuf, optbuflen, optp,
int ip6_pcbopt(int, u_char *, int, struct ip6_pktopts **, int, int);