Logout Redirect

Here's a patch for more flexible /caslogout redirect

1) allows for destination to be overriden in a standard Drupal way
e.g. make a link to "caslogout?destination=foobar" and the logout destination will be overridden
e.g. if redirection is turned off it can still be overridden with ?destination=

2) allows for redirect to be enabled without a specific destination set (i.e. destination_path is not mandatory and defaults to $base_url)
this is useful for multi-site installs where the user just wants to be redirected back to the site after logout
without requiring the administrator to needlessly enter the base URL as the redirect URL for each site.

Thanks.