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.

#986062 by Jeff Burnz. devel_generate_set_message() missing parameter 'type' in devel_generate_content().

#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 Fix "This form is outdated.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.