Improve secure randomness generation

[DL6ER]

What does this implement/fix?

Improve secure randomness generator by ensuring it retries automatically if interrupted by a signal plus printing a warning if the process will be blocking due to not enough entropy being available (this may delay FTL's startup for currently not externally visible reasons)

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Related issue or feature (if applicable): Context here https://discourse.pi-hole.net/t/unable-to-login-after-v6-update-getrandom-error/76269

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

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1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
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:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/unable-to-login-after-v6-update-getrandom-error/76269/15

[yubiuser]

This might need a rebase because it contains commits from #2238

The merge-base changed after approval.

[DL6ER]

Also assuming here that the approvals still hold after rebasing without any other changes.