Change 'debugexit' to quit main loop rather than exit(), add cleanup

A new envrionment variable GNOME_SHELL_ENABLE_CLEANUP is added which
causes us to attempt freeing global data.  The reason this isn't
enabled by default is that it's a waste of time at best, and at
worst in corner cases could cause crashes which would fill up
crash databases.  Better to leave it as a developer-only tool.

Start stubbing out some cleanup in ShellGlobal.

https://bugzilla.gnome.org/show_bug.cgi?id=649517

src/main.c

@@ -453,6 +453,7 @@ main (int argc, char **argv)
 {
   GOptionContext *ctx;
   GError *error = NULL;
+  int ecode;
 
   g_type_init ();
 
@@ -507,5 +508,13 @@ main (int argc, char **argv)
   /* Initialize the global object */
   shell_global_get ();
 
-  return meta_run ();
+  ecode = meta_run ();
+
+  if (g_getenv ("GNOME_SHELL_ENABLE_CLEANUP"))
+    {
+      g_printerr ("Doing final cleanup...\n");
+      g_object_unref (shell_global_get ());
+    }
+
+  return ecode;
 }