* sys/netinet6/{nd6.c,nd6_nbr.c,nd6.h}: implement garbage   correction for stale NDP entry which described in RFC2461   5.3. Each entry will removed in 1day in default.
* sys/netinet6/{nd6.c,nd6_nbr.c,nd6.h}: implement garbage
  correction for stale NDP entry which described in RFC2461
  5.3. Each entry will removed in 1day in default.

MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame