zcash_client_backend: Allow disabling Tor directory permissions tightening

[str4d]

Arti does some custom permissions tightening on its Tor directory. We've been having some problems on mobile devices related to permissions, and we believe that it is related to this. Given that mobile OSs provide sandboxing, there's not a huge benefit to extra permissions shenanigans (unlike on a desktop or laptop). So we should alter the create method to allow disabling the permissions tightening.