util: split platform-specific NetworkMonitor backends into separate files

In preparation for a major overhaul of the rtnetlink implementation.

Change-Id: Iab660ab4bceb0f06db01a31f264cdfe0b7cdf077
Refs: #3353
diff --git a/src/util/detail/network-monitor-impl-rtnl.hpp b/src/util/detail/network-monitor-impl-rtnl.hpp
new file mode 100644
index 0000000..141554d
--- /dev/null
+++ b/src/util/detail/network-monitor-impl-rtnl.hpp
@@ -0,0 +1,53 @@
+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/**
+ * Copyright (c) 2013-2016 Regents of the University of California.
+ *
+ * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
+ *
+ * ndn-cxx library is free software: you can redistribute it and/or modify it under the
+ * terms of the GNU Lesser General Public License as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option) any later version.
+ *
+ * ndn-cxx library is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ * PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.
+ *
+ * You should have received copies of the GNU General Public License and GNU Lesser
+ * General Public License along with ndn-cxx, e.g., in COPYING.md file.  If not, see
+ * <http://www.gnu.org/licenses/>.
+ *
+ * See AUTHORS.md for complete list of ndn-cxx authors and contributors.
+ */
+
+#ifndef NDN_UTIL_NETWORK_MONITOR_IMPL_RTNL_HPP
+#define NDN_UTIL_NETWORK_MONITOR_IMPL_RTNL_HPP
+
+#include "../network-monitor.hpp"
+
+#include <boost/asio/posix/stream_descriptor.hpp>
+
+namespace ndn {
+namespace util {
+
+const size_t NETLINK_BUFFER_SIZE = 4096;
+
+class NetworkMonitor::Impl
+{
+public:
+  Impl(NetworkMonitor& nm, boost::asio::io_service& io);
+
+private:
+  void
+  onReceiveRtNetlink(const boost::system::error_code& error, size_t nBytesReceived);
+
+private:
+  NetworkMonitor& m_nm;
+
+  uint8_t m_buffer[NETLINK_BUFFER_SIZE];
+  boost::asio::posix::stream_descriptor m_socket;
+};
+
+} // namespace util
+} // namespace ndn
+
+#endif // NDN_UTIL_NETWORK_MONITOR_IMPL_RTNL_HPP