Fix a logic error in circuit_stream_is_being_handled.

When I introduced the unusable_for_new_circuits flag in
62fb209d837f3f551, 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.
Merge branch 'bug10456' into maint-0.2.4
Merge remote-tracking branch 'origin/maint-0.2.4'