Change ownership of all shared memory objects before switching user

[DL6ER]

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Change ownership of all shared memory objects to the user/group configured in /etc/dnsmasq.d/01-pihole.conf. We already do this for pihole-FTL.log and pihole-FTL.db but forgot to do this for the shm objects.

[PromoFaux]

Approved based on #569 (comment)