Explore behavior of shutdown when shell commands are running

[jjb]

• https://github.com/jjb/ruby-clock/commit/300de3f621bd72fcd106a12500b8c3055545329d
• https://stackoverflow.com/questions/69653842/why-does-the-presence-of-a-forked-process-in-a-thread-change-the-behavior-of-sle
• https://gist.github.com/lpar/1032297
• https://github.com/kamstrup/command_runner_ng
• https://www.ruby-forum.com/t/executing-system-commands-in-threads-under-ruby-1-8-6/175066
• https://saveriomiroddi.github.io/Executing-and-killing-ruby-parallel-background-jobs/

[jjb]

https://github.com/jjb/ruby-clock/wiki/Ideas