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/Main.cxx

@@ -67,6 +67,10 @@
 #include "config/ConfigOption.hxx"
 #include "Stats.hxx"
 
+#ifdef ENABLE_NEIGHBOR_PLUGINS
+#include "neighbor/Glue.hxx"
+#endif
+
 #ifdef ENABLE_INOTIFY
 #include "db/update/InotifyUpdate.hxx"
 #endif
@@ -394,6 +398,19 @@ int mpd_main(int argc, char *argv[])
 
 	instance = new Instance();
 
+#ifdef ENABLE_NEIGHBOR_PLUGINS
+	instance->neighbors = new NeighborGlue();
+	if (!instance->neighbors->Init(io_thread_get(), *instance, error)) {
+		LogError(error);
+		return EXIT_FAILURE;
+	}
+
+	if (instance->neighbors->IsEmpty()) {
+		delete instance->neighbors;
+		instance->neighbors = nullptr;
+	}
+#endif
+
 	const unsigned max_clients = config_get_positive(CONF_MAX_CONN, 10);
 	instance->client_list = new ClientList(max_clients);
 
@@ -460,6 +477,12 @@ int mpd_main(int argc, char *argv[])
 
 	io_thread_start();
 
+#ifdef ENABLE_NEIGHBOR_PLUGINS
+	if (instance->neighbors != nullptr &&
+	    !instance->neighbors->Open(error))
+		FatalError(error);
+#endif
+
 	ZeroconfInit(*main_loop);
 
 	player_create(instance->partition->pc);
@@ -523,6 +546,13 @@ int mpd_main(int argc, char *argv[])
 	listen_global_finish();
 	delete instance->client_list;
 
+#ifdef ENABLE_NEIGHBOR_PLUGINS
+	if (instance->neighbors != nullptr) {
+		instance->neighbors->Close();
+		delete instance->neighbors;
+	}
+#endif
+
 	const clock_t start = clock();
 	DatabaseGlobalDeinit();
 	FormatDebug(main_domain,