Add a BF_PINNED block flag, and attach it to blocks containing pinned
objects (in addition to the usual BF_LARGE).

In heapCensus, we now ignore blocks containing pinned objects, because
they might contain gaps, and in any case it isn't clear that we want
to include the whole block in a heap census, because much of it might
well be dead.  Ignoring it isn't right either, though, so this patch
just fixes the crash and leaves a ToDo.