Cancel benchmarking if benchexec receives SIGTERM (signal 15)
Cancel benchmarking if benchexec receives SIGTERM (signal 15)

This makes SIGTERM and Ctrl+C (SIGINT) both cancel all runs.

We ignored SIGTERM for some reason when we had less robust process
handling, but there is not reason for this anymore.
Both runexec and containerexec already cancel the run on SIGTERM.

ultimate-pa/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,dbeyer/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec