Wait on the backgrounded rdctl process

[ericpromislow]

Fixes #4517 (although most likely a first draft):

An alternative would be to write the PID for the rdctl bg process to a temp file, and then read it during shutdown and wait if necessary. I'll go there if this PR is on the right track but we don't want to require every single first-test after running rdctl start to call wait_for_rdctl_background_process

[jandubois]

PR has merge conflicts, and needs to be rebased on main to run the "Spelling" check

[jandubois]

I don't understand how this PR works.

Does it work on macOS at all? There it just runs /usr/bin/open -a ... and exits; not directly starting a background process, but letting launchd do it. In that case $! is the empty string.

And if rdctl start is really waiting for the child process to exit, how does waiting on the rdctl progress actually if you haven't yet triggered an app shutdown?

[ericpromislow]

Not taking this approach. The problem might be solved anyway.