nixos/users-groups: warn on ambiguous password settings

After 4b128008c5 it took me a while in a
test setup to find out why `root` didn't have the password anymore I
declared in my config.

Because of that I got reminded how the order of preference works for the
password options:

    hashedPassword > password > hashedPasswordFile

If the user is new, initialPassword & initialHashedPassword are also
relevant. Also, the override is silent in contrast to any other
conflicting definition in NixOS.

To make this less surprising I decided to warn in such a case -
assertions would probably break too much that technically works as
intended.

Also removed the `initialHashedPassword` for `root`. This would cause a
warning whenever you set something in your own config and a `!` is added
automatically by `users-groups.pl`.

`systemd-sysusers` also seems to implement these precedence rules, so
having the warning for that case also seems useful.

nixos/modules/config/users-groups.nix

@@ -649,7 +649,6 @@ in {
         home = "/root";
         shell = mkDefault cfg.defaultUserShell;
         group = "root";
-        initialHashedPassword = mkDefault "!";
       };
       nobody = {
         uid = ids.uids.nobody;
@@ -897,7 +896,26 @@ in {
     ));
 
     warnings =
-      builtins.filter (x: x != null) (
+      flip concatMap (attrValues cfg.users) (user: let
+        unambiguousPasswordConfiguration = 1 >= length (filter (x: x != null) ([
+          user.hashedPassword
+          user.hashedPasswordFile
+          user.password
+        ] ++ optionals cfg.mutableUsers [
+          # For immutable users, initialHashedPassword is set to hashedPassword,
+          # so using these options would always trigger the assertion.
+          user.initialHashedPassword
+          user.initialPassword
+        ]));
+      in optional (!unambiguousPasswordConfiguration) ''
+        The user '${user.name}' has multiple of the options
+        `hashedPassword`, `password`, `hashedPasswordFile`, `initialPassword`
+        & `initialHashedPassword` set to a non-null value.
+        The options silently discard others by the order of precedence
+        given above which can lead to surprising results. To resolve this warning,
+        set at most one of the options above to a non-`null` value.
+      '')
+      ++ builtins.filter (x: x != null) (
         flip mapAttrsToList cfg.users (_: user:
         # This regex matches a subset of the Modular Crypto Format (MCF)[1]
         # informal standard. Since this depends largely on the OS or the