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

include/section_init.inc.php

@@ -749,7 +749,7 @@ if ( 'categories'==$page['section'] and isset($page['category']) )
       );
     if (!headers_sent())
     { // this is a permanent redirection
-      set_status_header(302);
+      set_status_header(301);
       redirect_http( $redirect_url );
     }
     redirect( $redirect_url );