Please review rt46340.

We'll need to discuss when it would be okay to commit this. It's a functional
change (though not on Windows, where filter-aaaa has been compiled in
since always), and we're past feature freeze for 9.12, so normally
it would have to wait until 9.13, unless we have a good reason to merge
it sooner.

In this case, though, the risk is *so* low - almost zero; it's not as if
this code hasn't been thoroughly tested - that in my opinion, reducing
the QA burden is a good enough reason. 9.13 is a long time to wait.