merge r1955 from branch-1_7 to trunk:

flat view small improvements (picture page on root category sorter urls and works in several cases)
correction in permalinks admin (error was assignment in if instead of comparison)

git-svn-id: http://piwigo.org/svn/trunk@1956 68402e56-0260-453c-a942-63ccdbb3a9ee

include/functions_url.inc.php

@@ -246,6 +246,10 @@ function make_picture_url($params)
     default:
       $url .= $params['image_id'];
   }
+  if ( !isset($params['category'] ) )
+  {// make urls shorter ...
+    unset( $params['flat'] ); 
+  }
   $url .= make_section_in_url($params);
   $url = add_well_known_params_in_url($url, $params);
   return $url;