Do not restart FTL while `pihole -g` is still ongoing

[DL6ER]

What does this implement/fix?

Do not try to restart FTL when a streamed pihole -g run is still ongoing. This to prevent being stuck in a situation where FTL has shut down 99% and cannot serve DNS until pihole -g finished running.

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Related issue or feature (if applicable): Fixes #2395

Pull request in docs with documentation (if applicable): N/A

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2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• [x] The code change is tested and works locally.
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