Don't execute runner when interrupted during sleep.
Don't crash when runners produce no output.
Try parsing the output of runners even when they exit non-zero.
Convert SIGTERM into SIGINT so shutdown logic is applied when killed.
Support graceful shutdown upon receiving SIGUSR1.

This mode will make netperf-wrapper gracefully interrupt an ongoing test
upon receiving a SIGUSR1, and parse the output generated by all runners
so far. The use case is running a test where the duration is not known
in advance, but results are still desired after the test is interrupted.