sosy-lab/benchexec: Release 1.21

This release contains only a few bug fixes:

• Forwarding signals to the benchmarked process (and thus, stopping runs via Ctrl+C), was broken on Python 2.
• If the freezer cgroup was available but mounted in a separate hierarchy, it was not used reliably as protection against fork bombs when killing processes.
• Since BenchExec 1.19, an exception would occur if a non-existing command was started in container mode.
• Since BenchExec 1.19, copying output files from a container would occur while subprocesses are still running and would be counted towards the walltime limit. This is fixed, although subprocesses will still be running if the freezer cgroup is not available (cf. #433).