- add an auto_login_key in users_table
- $conf['session_length'] is no more useful
and sessions length will be 0 (until browser closed)
- add $conf['remember_me_name'] for cookie remember name
git-svn-id: http://piwigo.org/svn/trunk@1493 68402e56-0260-453c-a942-63ccdbb3a9ee