ds: avoid excessive queueing when reaping PIDs

ds: avoid excessive queueing when reaping PIDs

We should not enqueue reap_pids() to run more than once per
EventLoop iteration.  We'll start reformatting reap_pids
to tabs, too, since we're no longer Danga::Socket.

We should also be able to remove timer usage for reaping
down-the-line once we stop abusing dwaitpid() in -watch.