Add skeleton of Linux CP Netlink Listener

Register lcp_nl_init() which adds interface pair add/del callbacks.

lcb_nl_pair_add_cb: Initiate netlink listener for first interface in its
  netns. If subsequent adds are in other netns, issue a warning. Keep
  refcount.
lcb_nl_pair_del_cb: Remove listener when the last interface pair is
  removed.

Socket is opened, file is added to VPP's epoll, with lcp_nl_read_cb()
and lcp_nl_error_cb() callbacks installed.
- lcp_nl_read_cb() calls lcp_nl_callback() which pushes netlink messages
  onto a queue and issues NL_EVENT_READ event, any socket read error
  issues NL_EVENT_READ_ERR event.
- lcp_nl_error_cb() simply issues NL_EVENT_READ_ERR event.

Then, initialize a process node called lcp_nl_process(), which handles:
- NL_EVENT_READ and call lcp_nl_process_msgs()
  - if messages are left in the queue, reschedule consumption after M
    msecs. This allows new netlink messages to continuously be read from
    the kernel, even if we have lots of messages to consume.
- NL_EVENT_READ_ERR and close/reopens the netlink socket.

lcp_nl_process_msgs() processes up to N messages and/or for up to M
msecs, whichever comes first. For each, calling lcp_nl_dispatch().

lcp_nl_dispatch() ultimately just throws the message away after
logging it with format_nl_object()

lcpng_netlink.h

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2021 Cisco and/or its affiliates.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <vlib/vlib.h>
+#include <plugins/lcpng/lcpng.h>
+
+#include <netlink/msg.h>
+#include <netlink/netlink.h>
+#include <netlink/socket.h>
+#include <netlink/route/link.h>
+#include <netlink/route/route.h>
+#include <netlink/route/neighbour.h>
+#include <netlink/route/addr.h>
+#include <netlink/route/link/vlan.h>
+
+typedef enum nl_event_type_t_
+{
+  NL_EVENT_READ,
+  NL_EVENT_READ_ERR,
+} nl_event_type_t;
+
+#define NL_RX_BUF_SIZE_DEF    (1 << 27) /* 128 MB */
+#define NL_TX_BUF_SIZE_DEF    (1 << 18) /* 256 kB */
+#define NL_BATCH_SIZE_DEF     (1 << 11) /* 2048 */
+#define NL_BATCH_DELAY_MS_DEF 50	/* 50 ms, max 20 batch/s */
+
+#define NL_DBG(...)    vlib_log_debug (lcp_nl_main.nl_logger, __VA_ARGS__);
+#define NL_INFO(...)   vlib_log_info (lcp_nl_main.nl_logger, __VA_ARGS__);
+#define NL_NOTICE(...) vlib_log_notice (lcp_nl_main.nl_logger, __VA_ARGS__);
+#define NL_WARN(...)   vlib_log_warn (lcp_nl_main.nl_logger, __VA_ARGS__);
+#define NL_ERROR(...)  vlib_log_err (lcp_nl_main.nl_logger, __VA_ARGS__);
+
+/* struct type to hold context on the netlink message being processed.
+ */
+typedef struct nl_msg_info
+{
+  struct nl_msg *msg;
+  f64 ts;
+} nl_msg_info_t;
+
+typedef struct lcp_nl_netlink_namespace
+{
+  struct nl_sock *sk_route;
+  nl_msg_info_t *nl_msg_queue;
+  uword clib_file_index;    // clib file that holds the netlink socket for this
+			    // namespace
+  u32 clib_file_lcp_refcnt; // number of interfaces watched in the this netlink
+			    // namespace
+  u8 netns_name[LCP_NS_LEN]; // namespace name (can be empty, for 'self')
+} lcp_nl_netlink_namespace_t;
+
+typedef struct lcp_nl_main
+{
+  vlib_log_class_t nl_logger;
+  /* TODO(pim): nl_ns should become a list, one for each unique namespace we
+   * created LCP pairs in.
+   */
+  lcp_nl_netlink_namespace_t nl_ns;
+
+  u32 rx_buf_size;
+  u32 tx_buf_size;
+  u32 batch_size;
+  u32 batch_delay_ms;
+
+} lcp_nl_main_t;
+
+static void lcp_nl_open_socket (u8 *ns);
+static void lcp_nl_close_socket (void);
+
+u8 *format_nl_object (u8 *s, va_list *args);
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */