Disable default fallback directories when DirAuthorities, AlternateDirAuthority, or FallbackDir are set

Currently, when we set DirAuthorities or AlternateDirAuthority, we don't add the default directory authorities. But, (as long as FallbackDir isn't set) we do add the default fallback directories.

It doesn't make sense to me to add the default fallback directories when we have custom DirAuthorities or AlternateDirAuthority. I think we should only add the default fallback directories when other directories are also set to their defaults.

However, the list of default fallback directories is currently NULL, so this issue currently has no effect.

I also can't imagine any scenarios where it would be useful to set an AlternateDirAuthority or DirAuthorities, and still get the default FallbackDir.

I can imagine this causing similar issues to #13163 (moved), where the default authorities were added to a custom set of authorities in some circumstances.

I'll create a patch to fix this, but it won't actually change tor's observable behaviour until we add directories to the default fallback directory list.

Bugfix on 90f6071d8dc0 in 0.2.4.7-alpha.