Systemd as cgroups manager

[dtrudg]

Describe the solution you'd like

Follow up after #60 / #292

systemd is the default init for all of our target distributions, and with cgroups v2 it would be useful to use it for cgroups management. This would allow cgroups applied to containers to fit neatly into the user slice / session scopes in the systemd managed v2 hierarchy.

Note that users can call systemd-run ... singularity ... to achieve something similar, and resource management is usually via a scheduler, so this may not be a high priority.

[dtrudg]

This will be a step toward #300