404 pages rendreed within layout

Before this pull request, the 404 and 404-search templates where handled
as full HTML pages without the page context (layout).

With this pull request both pages are rendered inside the selected
themes layout. Additionally the 404 template now contains the "search
form" and the homepage link.

All Themes are already updated with the above changes.

system/includes/functions.php

@@ -654,13 +654,7 @@ function get_keyword($keyword, $page, $perpage)
         }
     }
 
-    if (empty($tmp)) {
-        // a non-existing page
-        render('404-search', null, false);
-        die;
-    }
-
-    return $tmp = get_posts($tmp, $page, $perpage);
+    return ($tmp) ? get_posts($tmp, $page, $perpage) : [];
 }
 
 // Get related posts base on post tag.
@@ -1543,7 +1537,13 @@ EOF;
 function not_found()
 {
     header($_SERVER["SERVER_PROTOCOL"] . " 404 Not Found");
-    render('404', null, false);
+    render('404', array(
+        'title' => 'This page doesn\'t exist! - ' . blog_title(),
+        'description' => '',
+        'canonical' => false,
+        'breadcrumb' => '<a href="' . site_url() . '">' . config('breadcrumb.home') . '</a> &#187; 404 Not Found',
+        'bodyclass' => 'error-404',
+    ));
     die();
 }