Squashed commit of the following:

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.

include/nuttx/net/neighbor.h

@@ -0,0 +1,121 @@
+/****************************************************************************
+ * include/nuttx/net/neighbor.h
+ * Definitions for use with IPv6 Neighor Table
+ *
+ *   Copyright (C) 2016 Gregory Nutt. All rights reserved.
+ *   Author: Gregory nutt <gnutt@nuttx.org>
+ *
+ * Includes some definitions that a compatible with the LGPL GNU C Library
+ * header file of the same name.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name NuttX nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+#ifndef __INCLUDE_NUTTX_NET_NEIGHBOR_H
+#define __INCLUDE_NUTTX_NET_NEIGHBOR_H
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include <sys/types.h>
+#include <stdint.h>
+#include <net/ethernet.h>
+
+#include <nuttx/net/ip.h>
+#include <nuttx/net/netdev.h>
+
+#ifdef CONFIG_NET_IPv6
+
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+#ifndef CONFIG_NET_IPv6_NCONF_ENTRIES
+#  define CONFIG_NET_IPv6_NCONF_ENTRIES 8
+#endif
+
+/****************************************************************************
+ * Public Types
+ ****************************************************************************/
+
+/* Describes the link layer address */
+
+struct neighbor_addr_s
+{
+  uint8_t                    na_lltype;  /* See enum net_lltype_e */
+  uint8_t                    na_llsize;  /* Link layer header size */
+
+  union
+  {
+#ifdef CONFIG_NET_ETHERNET
+    struct ether_addr        na_ethernet;
+#endif
+#ifdef CONFIG_NET_6LOWPAN
+    struct netdev_maxaddr_s  na_sixlowpan;
+#endif
+  } u;
+};
+
+/* This structure describes on entry in the neighbor table.  This is intended
+ * for internal use within the Neighbor implementation.
+ *
+ * An unused entry can be detected by all zero entries, particularly the
+ * ne_ipaddr (the IPv6 unspecified address) which should never be zero for a
+ * real neighbor.
+ */
+
+struct neighbor_entry_s
+{
+  net_ipv6addr_t         ne_ipaddr;  /* IPv6 address of the Neighbor */
+  struct neighbor_addr_s ne_addr;    /* Link layer address of the Neighbor */
+  clock_t                ne_time;    /* For aging, units of tick */
+};
+
+#ifdef __cplusplus
+#define EXTERN extern "C"
+extern "C"
+{
+#else
+#define EXTERN extern
+#endif
+
+/****************************************************************************
+ * Public Function Prototypes
+ ****************************************************************************/
+
+#undef EXTERN
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* CONFIG_NET_LOOPBACK */
+#endif /* __INCLUDE_NUTTX_NET_NEIGHBOR_H */