privacy: Stream isolation for Tor

According to Tor's extensions to the SOCKS protocol
(https://gitweb.torproject.org/torspec.git/tree/socks-extensions.txt)
it is possible to perform stream isolation by providing authentication
to the proxy. Each set of credentials will create a new circuit,
which makes it harder to correlate connections.

This patch adds an option, -proxyrandomize (on by default) that randomizes
credentials for every outgoing connection, thus creating a new circuit.

2015-03-16 15:29:59 SOCKS5 Sending proxy authentication 3842137544:3256031132