choose_good_entry_server() is no longer used to choose entry guards

choose_good_entry_server() says:

 * If <b>state</b> is NULL, we're choosing a router to serve as an entry
 * guard, not for any particular circuit.

But in 0.3.0 and later, we use guardsets to choose entry guards. (And we use this function to choose individual entries.)

We should:

• fix the comment,
• check for NULL state (and do what?), and
• make sure that guard selection and entry selection doesn't get out of sync. (I think we're ok here, but I don't understand the new guard code very well.)