ws: Ignore broken pipe errors when sending http errors

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Be const correct in cockpit_web_server_return_content()

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Fix logout to use cockpit_web_server_return_content()

Instead of generating its own HTTP response.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>