When we started RefuseUnknownExits back in 0.2.2.11-alpha, we
started making exits act like they cache directory info (since they
need an up-to-date idea of who is really a router).  But this
included fetching needless (unrecognized) authorities' certs, which
doesn't make any sense for them.

This is related to, but not necessarily the same as, the issue that
Ian reported for bug #2297.

(This patch is based on a patch from a user who I believe has asked
not to be named.  If I'm wrong about that, please add the
appropriate name onto the changelog.)