(cherry picked from commit 6286aff5)

Avoid "Notice: Undefined property: stdClass::$timestamp in _drupal_session_write()" watchdog entry when switching users.

Remove unused devel/source page callback. Theme developer can implement its own if it wants to keep this feature.

'restrict access' => TRUE for various permissions. See http://drupal.org/update/modules/6/7#permissions_restrict_access

Suppress "Cannot modify header information - headers already sent" warning if we caused it ourselves.

#992938 by Dave Reid. Fix the switch user block should use format_username(), which requires full user objects.

#970688 by Mark Trapp, salvis: Use error_reporting() in the backtrace_error_handler() to allow setting the desired error flags in settings.php or php.ini.

915620 by salvis. Fix "This form is outdated. Reload the page and try again." error in Switch User function

#568758 by Mark Trapp | donquixote: Added Devel backtraces should follow Drupal core's handling of E_DEPRECATED.

Attempt to support the PEAR version of Firephp. Untested, but does not break downloaded version (thats tested).