Log more details around child process start and stop

[github-actions]

This is an automatic cherry-pick of f122725cca4f4db138f4e78e52932b5094185fcf

The original PR was #182 with message:

By logging the PID, it's easier to tell sessions apart.

The log level has been reduced to log_debug,
since there are log_info messages that better convey the state transitions in the applications.

Exit code and signal number are no longer mixed up in the log. The new log messages look like this:

• killall -SIGINT dockerd → "exited with exit code 0"
• killall -SIGTERM dockerd → "exited with exit code 0"
• killall -SIGABRT dockerd → "exited with exit code 2"
• killall -SIGKILL dockerd → "was killed by signal 9"

Checklist before requesting a review

• I have performed a self-review of my own code
• I have verified that the code builds perfectly fine on my local system
• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have verified that my code follows the style already available in the repository
• I have made corresponding changes to the documentation