Call the isc__nm_failed_connect_cb() early when shutting down

Call the isc__nm_failed_connect_cb() early when shutting down

When shutting down, calling the isc__nm_failed_connect_cb() was delayed
until the connect callback would be called.  It turned out that the
connect callback might not get called at all when the socket is being
shut down.  Call the failed_connect_cb() directly in the
tlsdns_shutdown() instead of waiting for the connect callback to call it.