in6_addmulti
inm = in6_addmulti(&tunnel->t_dst6, ifp0, &error);
sc->sc_inmulti = in6_addmulti(&sc->sc_dst.in6, ifp0, &error);
imm->i6mm_maddr = in6_addmulti(addr, ifp, errorp);
struct in6_multi *in6_addmulti(const struct in6_addr *, struct ifnet *, int *);
in6_addmulti(&llsol, ifp, &error);