net/netlink/netlink_route.c: Add Netlink socket NETLINK_ROUTE support for getting a snopshot of the Neighbor table.
net/neighbor/neighbor_snapshot.c: Add neighbor_snapshot() that will eventually be used by the Netlink sockets. Also fixed naming violation 'struct neighbor_entry' -> 'struct neighbor_entry_s'.
include/nuttx/net/neighbor.h: Expose format of the IPv6 neighbor table for use with Netlink sockets.
