Motivation for this change

First 9 commits are just module cleanup and should be squashed after review. By reading commit by commit it is easy to see I didn't really change anything, just cleanup.
Final commit should remain separate and provides an easy way for users to decide if they want to run httpd as a non root user by simply adding this to their configuration:

systemd.services.httpd = {
  serviceConfig.User = lib.mkForce "wwwrun";
  AmbientCapabilities = "CAP_NET_BIND_SERVICE";
};

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.