permalink and category.php redirections are permanent (http code 301) instead of temporary (301)

added rel nofollow for the metadata link on picture page
changed h2 background color style in dark/mail-css (was not readable when images are not downloaded)

git-svn-id: http://piwigo.org/svn/branches/branch-1_7@1949 68402e56-0260-453c-a942-63ccdbb3a9ee

category.php

@@ -60,7 +60,7 @@ if ( isset($_GET['cat']) )
 $url = make_index_url($url_params);
 if (!headers_sent())
 {
-  set_status_header(302);
+  set_status_header(301);
   redirect_http( $url );
 }
 redirect ( $url );