Add sigrtmin option

[jacklul]

What does this PR aim to accomplish?:

In some cases kill -RTMIN can send incorrect signal value to the FTL causing it to terminate instead of performing lists reload.
#2573

How does this PR accomplish the above?:

Undocumented sigrtmin action is added which returns the value of SIGRTMIN FTL expects
That value can be used by the pihole script to send the correct signal

PR on the script side: pi-hole/pi-hole#6358

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[DL6ER]

Thank you.

On Ubuntu 24.04:

$ pihole-FTL sigrtmin
34

On Alpine Linux 3.21:

$ pihole-FTL sigrtmin
35

[yubiuser]

Should we amend this doc?

https://docs.pi-hole.net/ftldns/signals/

[DL6ER]

Well, the docs are already circumventing this by using exact numbers:

But this is, of course, only correct for the pre-built binaries and those built under musl compilers (e.g. in the devcontainer). Everyone building this locally on Debian, Ubuntu, Fedora, ... will have glibc so the numbers in the docs are wrong. I tend to agree we should amend this. Feel free to do it if you have a spare minute, I'm not sure when I can sit down and do it myself.

Probably something like:

The numbers in parentheses assume RTMIN=35, you can verify this on your particular system using the pihole-FTL ...

[yubiuser]

pi-hole/docs#1270