When I introduced the unusable_for_new_circuits flag in
62fb209d83, I had a spurious ! in the
circuit_stream_is_being_handled loop. This made us decide that
non-unusable circuits (that is, usable ones) were the ones to avoid,
and caused it to launch a bunch of extra circuits.
Fixes bug 10456; bugfix on 0.2.4.12-alpha.
