Improve why3-cpulimit so that it forcibly kills processes after a while.
A child process (e.g. CVC4) might catch SIGXCPU. If it gets stuck then, it won't consume any additional cpu time, so the system won't forcibly kill it. So why3-cpulimit has to kill it. Note that, if the system is overloaded, why3-cpulimit might kill the child process before it has even reached its cpu time limit. Hopefully, the 60' additional time will suffice in practice.
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