Allow parallel FTL processes

[DL6ER]

What does this implement/fix?

So far, FTL tried to prevent multiple processes from being started on the same machine. However, as the pihole-FTL binary became kind of a Swiss-army-knife offering integrated features such as sqlite3, sqlite3_rsync, sha256sum, gzip, lua, dhcp-discover, and others (like a small TLS certificate managing toolset), it becomes more and more likely that there are long-lived interactive pihole-FTL instances on the machine. They may coexist without any issues, however, once such an interactive process is running, restarting the original pihole-FTL may be prevented under certain circumstances.

Hence, I propose going a different, much easier path: Allow parallel processes to coexist peacefully much like most other daemons do. Duplicate processes will in the end not be able to fully start up as they cannot acquire the DNS and webserver ports. However, the currently possible situation of not being able to restart the main DNS-providing process is resolved hereby.

We allow the new parallelism by ensuring uniqueness of the shard memory objects by changing their name to include the main PID of each FTL process, e.g., FTL-34532-clients instead of (right now) the shorter FTL-clients.

No change to the post-stop command is needed as the deletion glob still matches the tweaked names:

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Related issue or feature (if applicable): N/A

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

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[pralor-bot]

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

https://discourse.pi-hole.net/t/i-cant-reach-the-pi-hole-v6-web-interface/75639/27

[yubiuser]

This PR nicely fixes the issue with long-running interactive FTL processes parallel with a 'main' FTL doing the DNS work. However, it does not prevent users from manually starting a second 'full' FTL wich will only start some components and runs as kind of zombie in the background

Maybe we could only allow one main FTL process and all other allowed FTL processes need to be called with a specific sub-process command (e.g. sqlite3, sha256sum,....)?

[DL6ER]

Closing in favor of #2184