Release 3.17

• Even more robust handling of child cgroups created within a run
  Despite the improvements from BenchExec 3.13 there could be crashes because sometimes cgroups vanish while we iterate over them. Avoiding this did not fully work, maybe due to some delays in the kernel, so now we handle this.
• Actually fix handling of non-printable characters in environment variables.
  The fix that was part of BenchExec 3.12 only worked if a non-printable character was the first character of the value of the environment variable.