mike is concerned that we would get too much exposure to adversaries,
if we enforce that none of our L2 guards can be in the same family.
this change set now essentially finishes the feature that commit a77727cdc
was attempting to add, but strips the "_and_family" part of that plan.
We had omitted some checks for whether our vanguards (second layer
guards from proposal 333) overlapped or came from the same family.
Now make sure to pick each of them to be independent.
Fixes bug 40639; bugfix on 0.4.7.1-alpha.