make bereq.uncacheable read-only outside backend_fetch

Writing to bereq.uncacheable in backend_response and backend_error would only
make sense in the context of a retry. In most real world cases, writing to
be*resp*.uncacheable would be intended in backend_response and backend_error.

lib/libvcc/generate.py

@@ -359,7 +359,7 @@ sp_variables = [
 	('bereq.uncacheable',
 		'BOOL',
 		( 'backend', ),
-		( 'backend', ), """
+		( 'backend_fetch', ), """
 		Indicates whether the object requested from the backend is
 		going to be uncacheable - either because the request was an
 		explicit pass from the client side or a hit on an uncacheable