Be more aggressive about enabling Extended ORPort

Bridges without Extended ORPort do not publish statistics about usage. Some people really care about statistics.

In #9651 (moved) (merged in 0.2.5.1) we decided to add a warning if the user has PTs but no Extended ORPort.

Maybe we should be a bit more aggressive about enabling Extended ORPort, since many operators might simply ignore that warning.

Some solutions:

a) (Most aggressive) Just enable Extended ORPort by default if ORPort and PTs are in effect. Of course, make it listen only on localhost.

b) Turn the warning into an error, so that people can't start their bridge without it. The problem here is that it's not really an error, since the bridge will work fine without ExtORPort, but there will be no stats.

c) Use Unix sockets in platforms that support it; similar to how we do it for ControlPort.