lib: Attach stdout to child only if --log=stdout and stdout FD is a tty (backport #16738)

[mergify]

We've faced with a regression after we moved from frr 8.5 to a newer versions (9.0, 9.1, 10.0, 10.1).

In our automation we start frr from python code using subprocess like this:

subprocess.run(
    ["/usr/lib/frr/frrinit.sh", "start"],
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
)

This call hangs in poll() for stdout FD indefinitely and frrinit.sh hangs in zombie state unless we kill subprocess.run() call.
If we remove stdout capturing from subprocess.run() call (make associated to stdout FD a tty FD), things go well and process terminates correctly with no hangs and zombies.
This commit fixes this situation where stdout of a process started in a daemon mode was attached to a calling process regardless of presence --log option.

If this patch is acceptable, it's worth backporting down to 9.0 branch, where the initial commit 4d28aea appeared.

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This is an automatic backport of pull request #16738 done by Mergify.