Fix sched_stop_secondary_cpus() to properly drain CPUs

Fix sched_stop_secondary_cpus() to properly drain CPUs

TAILQ_FOREACH() isn't safe to use in sched_chooseproc() to iterate
over the run queues because within the loop body we remove the threads
from their run queues and reinsert them elsewhere.  As a result, we
end up only draining the first thread of each run queue rather than
all of them.

ok kettenis