Handle timeout more gracefully in background pings

Added a 2-second default timeout for XMLRPC/extended pings, configurable as [ping,timeout].
No longer repeating the entire ping section if we had an HTTP error during a submission.
For now, dropping the bad item and continuing on with others. (Todo: individual retry and cleaner discards of blacklisted broken-for-now sites.)

lib/default.php

@@ -188,7 +188,8 @@ $default =
         'cache' =>
         array('base' => null),
         'ping' =>
-        array('notify' => array()),
+        array('notify' => array(),
+              'timeout' => 2),
         'inboxes' =>
         array('enabled' => true), # ignored after 0.9.x
         'newuser' =>