neighbor: new subsystem to detect file servers on the local network

This commit adds the NeighborPlugin API which can be used to detect
nearby file servers that can be used by input plugins.  This list of
servers is exported using the new "listneighbors" command.  The idle
even "neighbor" notifies interested clients when a new neighbor is
found or an existing one is lost.

There's a lot missing currently: protocol&user documentation, and a
way to "mount" remote servers into the music database.  Obviously,
some code from the UPnP database plugin can be moved to a neighbor
plugin.

src/command/NeighborCommands.cxx

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "NeighborCommands.hxx"
+#include "client/Client.hxx"
+#include "Instance.hxx"
+#include "Main.hxx"
+#include "protocol/Result.hxx"
+#include "neighbor/Glue.hxx"
+#include "neighbor/Info.hxx"
+
+#include <set>
+#include <string>
+
+#include <assert.h>
+
+bool
+neighbor_commands_available()
+{
+	return instance->neighbors != nullptr;
+}
+
+CommandResult
+handle_listneighbors(Client &client,
+		     gcc_unused int argc, gcc_unused char *argv[])
+{
+	assert(instance->neighbors != nullptr);
+
+	const auto neighbors = instance->neighbors->GetList();
+	for (const auto &i : neighbors)
+		client_printf(client,
+			      "neighbor: %s\n"
+			      "name: %s\n",
+			      i.uri.c_str(),
+			      i.display_name.c_str());
+	return CommandResult::OK;
+}