Runit support for Incus (Closes: 1084990).

Here is my initial merge request for Runit support for Incus.

Please don't merge yet! It has not yet been build tested as I still have issues with building this locally. I hope once the pipeline has completed I may be able to download the package Salsa build 😄

I will comment on certain things in follow-up comments.

@abower, @Lorenzo.ru.g-guest and @gibmat please review this.

Current git commit message (will likely be updated before final merge):

All files are licensed under Apache-2.

Based on the Systemd unit and the init script, with the omission that is does not create "/var/log/incus" on each startup as that directory appears to be created at package installation.

This is still the legacy way of using dh_runit such as used in "openssh-server" package. I am willing to provide a merge request to change it to the new way which goes through "/usr/share/runit/sv" instead of installing the service directory directly into "/etc/sv" whenever it becomes available to use in this context.

Thanks to Lorenzo, Andrew and Mathias for review!