watchdog: check dead children aren't holding locks.

As well as the main watchdog loop, we need to do this
in kill_all_kids() as it can spin indefinitely.
watchdog: reap dead kids in the kill_all_kids loop

Again, because we're spinning here, we may need to do
periodic cleanup, like the watchdog main loop does.

[ A future cleanup might be to split out the common parts
  of the two 'spin indefinitely' loops to a single function. ]