systemd: Delegate cgroup management without turning on controllers.

[shlevy]

Nix currently doesn't do any resource control, and Delegate=yes turns on all the controllers.

In particular, this enables using cpusets with cgroups V1 alongside the Nix daemon.

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[Ericson2314]

#11414 c.f. this is when it was originally added.

[shlevy]

Before:

$ sudo cset set -c 0 cpu0
mount: /cpusets: none already mounted on /run/keys.
       dmesg(1) may have more information after failed mount system call.
cset: **> mount of cpuset filesystem failed, do you have permission?

After:

$ sudo cset set -c 0 cpu0
cset: --> created cpuset "cpu0"

[Mic92]

cc @parkerhoyes

[edolstra]

Looks good to me. I guess we'll have to change this back if Nix does start doing resource control (see e.g. #7388)?

[shlevy]

You can change it back to the list of specific controllers at least.