Increase time for follow-up SIGKILLs

Sending one at 30s and then at 32s (next time round the loop)
might not be a long enough delta if we're swapping like crazy
and taking a while to page stuff in/out. Bump it up to 40s.
watchdog: remove unnecessary sleep from child checking loop

We don't need to wait for a child to exit here, because we don't
interact with it any further in this function.