csrf: (re)generate secret on GUI restart

(cherry picked from commit d6411ae0)
(cherry picked from commit b78720b8)

src/etc/inc/system.inc

@@ -963,6 +963,7 @@ function system_webgui_start()
     global $config;
 
     chdir('/usr/local/www');
+    @unlink('/usr/local/www/csrf/csrf-secret.php');
 
     /* defaults */
     $portarg = "80";

src/www/csrf/csrf-magic.php

@@ -425,6 +425,8 @@ function csrf_get_secret()
     }
     if (is_writable($dir)) {
         $secret = csrf_generate_secret();
+        touch($file);
+        chmod($file, 0600);
         $fh = fopen($file, 'w');
         fwrite($fh, '<?php $secret = "'.$secret.'";' . PHP_EOL);
         fclose($fh);

src/www/csrf/csrf-secret.php

-<?php $secret = "3f27f3793cfb16c2700474d5c613487e258143d9";