be willing to bootstrap from all three of our directory guards

Also fix a bug where if the guard we choose first doesn't answer, we
would try the second guard, but once we connected to the second guard
we would abandon it and retry the first one, slowing down bootstrapping.

The fix in both cases is to treat all our initially chosen guards as
acceptable to use.

Fixes bug 9946.
Merge branch 'maint-0.2.4'