Handle TTL for blocked queries seperately

[DL6ER]

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So far, we used local-ttl=2 hard-coded in dnsmasq's config because this ensured clients won't cache blocked queries for longer times, possibly rendering whitelisting ineffective.

This PR implements a new TTL for blocked queries only. This allows users to set local-ttl to any value they prefer without this being overwritten in 01-pihole.conf after each update. This seems useful in context of locally used hostnames that are known to stay constant over long times (e.g., printers, etc.).

The new value defaults to 2 seconds (no change in behavior) and can be controlled using the new FTL config option BLOCK_TTL=2

[yubiuser]

Documentation needed.

[DL6ER]

See pi-hole/docs@60195ea in pi-hole/docs#573

[pralor-bot]

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

https://discourse.pi-hole.net/t/change-default-blacklist-ttl-via-environment-variables/48926/2