Unarchive no longer transfers archive if creates= check succeeds

Unarchive no longer transfers the archive in case the creates= parameter yields a successful match, thus preventing lenghty operations over the network (especially with large archive distributions).

Please note: the unpack module on the client is actually called twice to achieve this - with extra signalling using precheck=True/False parameter. Please suggest other form of signalling if this is not the correct way of doing it.