Warn if traffic accounting is enabled and likely to break anonymity
A Tor instance should log a warning if traffic accounting is enabled and any of the following are true:
- It is configured to operate two or more hidden services (e.g. there are two or more HiddenServiceDir lines in its configuration).
- It is configured to operate at least one hidden service and run as a relay or bridge.
This change attempts to address a possible user safety issue and can't break Tor in a subtle way, so I'm filing it for 0.2.3.x.