Session security improvement - user agent

[thekabal]

From https://stackoverflow.com/questions/5081025/php-session-fixation-hijacking#5081453 :

"Include the user agent from $_SERVER['HTTP_USER_AGENT'] in the session. Basically, when the session starts, store it in something like $_SESSION['user_agent']. Then, on each subsequent request check that it matches."

Non-trivial, but useful. Lets try to get this added.