set socks auth for stream isolation

People are using the Tor that comes with TBB for other stuff than Tor Browser traffic (onionshare etc.).

As long as legacy/trac#3455 (moved) is not implemented, please consider setting a default (or random?) socks auth, unless the TOR_SOCKS_AUTH env variable is set.

Tor would notice it thanks to IsolateSOCKSAuth and all Tor Browser traffic would go through its own circuit.