UI: alternate fix for T65702, handling of auto-saving userprefs

The behavior for loading factory settings wasn't clear for users.

This commit changes the behavior:

- Loading factory settings always disables auto-save
  for the current session.
- The internal setting to skip saving on exit is now exposed
  in the preferences (when enabled).
- The menu item "Load Factory Settings (Temporary)" has been removed
  since it's always temporary.

This way users can always reset factory settings without
having to consider the combination of options that might cause their
preferences to be overwritten at exit.

If they want to enable auto-save for the current session
this can be done from the preferences.

source/blender/python/intern/bpy_app.c

@@ -508,6 +508,12 @@ static PyGetSetDef bpy_app_getsets[] = {
      (char *)bpy_app_global_flag_doc,
      (void *)G_FLAG_EVENT_SIMULATE},
 
+    {(char *)"use_userpref_skip_save_on_exit",
+     bpy_app_global_flag_get,
+     bpy_app_global_flag_set,
+     (char *)bpy_app_global_flag_doc,
+     (void *)G_FLAG_USERPREF_NO_SAVE_ON_EXIT},
+
     {(char *)"binary_path_python",
      bpy_app_binary_path_python_get,
      NULL,