Use PHPs session_regenerte_id() instead of using (unreliable) mt_rand() function (#1486281)

CHANGELOG

@@ -1,6 +1,7 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Get around unreliable rand() and mt_rand() in session ID generation (#1486281)
 - Fix some emails are not shown using Cyrus IMAP (#1487820)
 - Fix handling of mime-encoded words with non-integral number of octets in a word (#1487801)
 - New config option for custom logo

program/include/rcube_session.php

@@ -212,20 +212,8 @@ class rcube_session
     $this->destroy(session_id());
     $this->vars = false;
 
-    $randval = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
-
-    for ($random = '', $i=1; $i <= 32; $i++) {
-      $random .= substr($randval, mt_rand(0,(strlen($randval) - 1)), 1);
-    }
-
-    // use md5 value for id
-    $this->key = md5($random);
-    session_id($this->key);
-
-    $cookie   = session_get_cookie_params();
-    $lifetime = $cookie['lifetime'] ? time() + $cookie['lifetime'] : 0;
-
-    rcmail::setcookie(session_name(), $this->key, $lifetime);
+    session_regenerate_id(false);
+    $this->key = session_id();
 
     return true;
   }