1, core-services: migrate towards core-services

[Vaelatern]

Based 100% on the work by @sbromberger

#113

I didn't want the work to die.

After this change, folks using lxd or incus containers will be able to create their own lower-priority core service to modify the permissions on the files to enable proper shutdown/reboot of the containers. These core services will survive upgrades to void-runit, unlike the current approach in the container script.