nscd service: fix ordering and start automatically

[abbradar]

During working on #150837 I discovered that google-oslogin test
started failing, and so did some of my development machines. Turns out
it was because nscd doesn't start by default; rather it's wanted by
NSS lookup targets, which are not always fired up.

To quote from systemd.special(7) section on nss-user-lookup.target:

All services which provide parts of the user/group database should be
ordered before this target, and pull it in.

Following this advice and comparing our unit to official sssd.service
unit (which is a similar service), we now pull NSS lookup targets from
the service, while starting it with multi-user.target.

Motivation for this change

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

Tested by using it together with new GCE configuration from #150837, and with google-oslogin test.

[arianvp]

Thanks for this. This is definitely correct and the previous behaviour was not. I also recently noticed nscd started way too late.

[arianvp]

It might actually make sense to start nscd even earlier than multi-user.target but for now this is fine

[abbradar]

If I understand this correctly, the idea is instead to make services which depend on name resolution run after NSS lookup targets are reached, so multi-user.target here should be fine.

[flokli]

Let's get this in. It's better than it was for sure, if we can improve further, we still can ;-)

[github-actions]

Successfully created backport PR #154620 for release-21.11.