Support for HttpOnly cookies.

Support for HttpOnly cookies.
HttpOnly cookies prohibit access by Javascript. Currently only IE6 supports
it. See http://msdn.microsoft.com/workshop/author/dhtml/httponly_cookies.asp
In order to achieve that the cookie with the session_id also contains the
httponly cookie attribute I introduced a new function, sqsession_start which
overwrites the cookie set by php by our own cookie containing the session_id
and the httponly attribute.
All session_start calls are replaced by sqsession_start.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@10174 7612ce4b-ef26-0410-bec9-ea0150e637f0