Add *if_nd to struct ifnet, call nd6_if{at,de}tach() directly

Add *if_nd to struct ifnet, call nd6_if{at,de}tach() directly

*if_afdata[] and struct domain's dom_if{at,de}tach() are only used with
IPv6 Neighbour Discovery in6_dom{at,de}tach(), which allocate/init and
free single struct nd_ifinfo.

Set up a new ND-specific *if_nd member directly to avoid yet another
layer of indirection and thus make the generic domain API obsolete.

The per-interface data is only accessed in nd6.c and nd6_nbr.c through
the ND_IFINFO() macro;  it is allocated and freed exactly once during
interface at/detach, so document it as [I]mmutable.

OK bluhm mvs claudio