reliability: check writing to stdout and stderr was successful

reliability: check writing to stdout and stderr was successful

This is expected behaviour.

$ mtr --help > /dev/full; echo $?
./mtr: write error: No space left on device
1

Earlier write errors returned successful zero.

Reference: https://www.irill.org/events/ghm-gnu-hackers-meeting/videos/jim-meyering-goodbye-world-the-perils-of-relying-on-output-streams-in-c