config: change default auth_timeout to 5 seconds

This partially addresses CVE-2014-3639.

This will change the default on the system bus where the limit
  <limit name="auth_timeout">...</limit>
is not specified.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=80919Reviewed-by: Thiago Macieira <thiago@kde.org>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

bus/config-parser.c

@@ -438,7 +438,7 @@ bus_config_parser_new (const DBusString      *basedir,
        * and legitimate auth will fail.  If interactive auth (ask user for
        * password) is allowed, then potentially it has to be quite long.
        */
-      parser->limits.auth_timeout = 30000; /* 30 seconds */
+      parser->limits.auth_timeout = 5000; /* 5 seconds */
       
       parser->limits.max_incomplete_connections = 64;
       parser->limits.max_connections_per_user = 256;