Cleanup if startup failed

[yubiuser]

What does this PR aim to accomplish?:

Cleanup files if pihole-FTL did not start properly after getting invoked by pihole-FTL.service.

With #4857 we added the exit code, however do not use ist so far. Now we do.

This is a re-implementation of #4887 after #4924 has been merged

How does this PR accomplish the above?:

Run the pihole-FTL-poststop.sh if pihole-FTL started with a non-zero exit code or we trap INT HUP TERM ABRT

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[MichaIng]

I cannot do reviews on this repo btw, but looks good 👍.

[pralor-bot]

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

https://discourse.pi-hole.net/t/pi-hole-web-v5-18-2-and-core-v5-15-1-released/60695/1