nixos/users-groups: check format of passwd entries

[alyssais]

Motivation for this change

Things will get quite broken if an /etc/passwd entry contains a
colon (which terminates a field), or a newline (which terminates a
record). I know because I just accidentally made a user whose home
directory path contained a newline!

So let's make sure that can't happen.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[lukegb]

I kinda wish there was a good way to write unit tests for small functions like this. Or maybe there is and I don't know about it?

[alyssais]

I kinda wish there was a good way to write unit tests for small functions like this. Or maybe there is and I don't know about it?

There's lib.runTests, but I don't know where we'd expose an attribute for a test for this in a way that a full test run would pick it up.

[sternenseemann]

On a related note, I was thinking about maybe extending the login test to also test login for an user which has been created by the nix infrastructure. Currently it only uses useradd which doesn't really reflect 99% of users in nixos.