added description strings for x_nat_t_new_mapping and x_migrate when printing stats of pfkey packets.
added description strings for x_nat_t_new_mapping and x_migrate when
printing stats of pfkey packets.

contributed by Francis Dupont.

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