Periodic writes to task-set-specific result files #1215

benchexec writes result files for each <rundefinition> and <tasks> tag in the benchmark definition. The former are written periodically during the benchmarking, whereas the latter are written only at the end, once all results are in. Since #1211 writing of the run-definition-specific result files is optional and one can use benchexec with only result files per task set (if using --results-per-taskset and not --results-per-rundefinition). In this more, no intermediate result files are written. This has the disadvantage that one cannot easily look at the partial results that have been obtained so far and that if benchexec crashes, all results are lost. So it would be better if the task-specific files would also be written incrementally (at least when they are the only produced result files).