Fixes related to cssname

A couple of changes I suggest to fix the load of pages when you have selected to load an specific cssname.
For example if the very first time you access the page with the URL:
index.php?cssname=template1.
When you logout and you are redirected to the authentication screen, you should see the exact same url, with the ?cssname=template1.
The same scenario when you have raised an error and there is a GO to login link.

The files changed are:
- common/lib/customer.module.access.php
- customer/PP_error.php