merge revisions 1938 and 1949 from branch-1_7 to trunk:

dark theme correction
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/trunk@1950 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 );