Ensure proper unprivileged user if pihole-FTL is started as root

[DL6ER]

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What does this PR aim to accomplish?:

Ensure that when pihole-FTL is launched as user root (it will drop permissions as soon as this is possible), it drops down to pihole:pihole instead of the default nobody:nogroup.

This is necessary as pihole-FTL needs to be able to periodically create new files - namely the SQLite3 rollback journals - in /etc/pihole. As user nobody, however, cannot do this, the current way of handling this leads to a failure of our long-term database implementation.

How does this PR accomplish the above?:

Use appropriate dnsmasq options to instruct the code to not go to nobody:nogroup but rather pihole:pihole.

[pralor-bot]

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

https://discourse.pi-hole.net/t/ftldns-upgrade-failed-solved/12022/48