1. CMSG_NXTHDR(mhdr, cmsg) is supposed to dereference cmsg and return
the next header in the chain. If cmsg is NULL it should return
the first header, behaving essentially like CMSG_FIRSTHDR().
2. inet6_rth_(space|init|add) should do basic checking on their input
to verify that the number of headers (segments) is
between 0 and 127 inclusive.
MFC-After: 1 month
18a6073100