bftw() implements depth-first search by appending files to a batch, then
prepending the batch to the queue. When we switched to separate file/
directory queues, this was only implemented for the file queue.
Unbuffered searches don't use the file queue, so they were all breadth-
first in practice.
This meant that iterative deepening (-S ids) was actually "iterative
deepening *breadth*-first search," a horrible strategy with no advantage
over regular breadth-first search. Now it performs iterative deepening
*depth*-first search, which at least limits its memory consumption.