update: added update_global_init() and update_global_finish()

Those two functions are called when MPD starts and exits.  It allows
the update library to perform global initialization and
deinitialization.  The implementations are currently empty.

src/main.c

@@ -416,6 +416,7 @@ int main(int argc, char *argv[])
 	initPermissions();
 	initPlaylist();
 	decoder_plugin_init_all();
+	update_global_init();
 
 	init_main_notify();
 
@@ -497,6 +498,7 @@ int main(int argc, char *argv[])
 	dc_deinit();
 	pc_deinit();
 	command_finish();
+	update_global_finish();
 	decoder_plugin_deinit_all();
 	music_pipe_free();
 	cleanUpPidFile();

src/update.c

@@ -580,3 +580,11 @@ void reap_update_task(void)
 		progress = UPDATE_PROGRESS_IDLE;
 	}
 }
+
+void update_global_init(void)
+{
+}
+
+void update_global_finish(void)
+{
+}

src/update.h

@@ -20,6 +20,10 @@
 #ifndef MPD_UPDATE_H
 #define MPD_UPDATE_H
 
+void update_global_init(void);
+
+void update_global_finish(void);
+
 unsigned
 isUpdatingDB(void);