nixos/networking: warn when both networkd and dhcpcd can collide

[adamcstephens]

Description of changes

I finally got around to digging into some networking issues I'd been seeing. Default routes disappearing, IPs getting removed from interfaces, etc. I discovered that I had not customized networking.useDHCP so it was still set to the default true, but I had enabled networkd. With this configuration combination, both networkd and dhcpcd will manage interfaces, causing havoc of course.

I would recommend changing the options in some way to potentially tie useNetworkd to systemd.network.enable, but currently useNetworkd enables the latter, so it is not a simple change. For now, I present a warning to assist my fellow nixers until a potentially better solution can be discussed.

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/)
• 23.11 Release Notes (or backporting 23.05 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
• Fits CONTRIBUTING.md.

[Gerg-L]

LGTM