nixos/users-groups: createHome: Ensure HOME permissions, fix description

configuration.nix(1) states

    users.extraUsers.<name>.createHome
        [...] If [...] the home directory already exists but is not
        owned by the user, directory owner and group will be changed to
        match the user.

i.e. ownership would change only if the user mismatched;  the code
however ignores the owner, it is sufficient to enable `createHome`:

    if ($u->{createHome}) {
        make_path($u->{home}, { mode => 0700 }) if ! -e $u->{home};
        chown $u->{uid}, $u->{gid}, $u->{home};
    }

Furthermore, permissions are ignored on already existing directories and
therefore may allow others to read private data eventually.

Given that createHome already acts as switch to not only create but
effectively own the home directory, manage permissions in the same
manner to ensure the intended default and cover all primary attributes.

Avoid yet another configuration option to have administrators make a
clear and simple choice between securely managing home directories
and optionally defering management to own code (taking care of custom
location, ownership, mode, extended attributes, etc.).

While here, simplify and thereby fix misleading documentation.

nixos/modules/config/update-users-groups.pl

@@ -209,10 +209,11 @@ foreach my $u (@{$spec->{users}}) {
         }
     }
 
-    # Create a home directory.
+    # Ensure home directory incl. ownership and permissions.
     if ($u->{createHome}) {
         make_path($u->{home}, { mode => 0700 }) if ! -e $u->{home};
         chown $u->{uid}, $u->{gid}, $u->{home};
+        chmod 0700, $u->{home};
     }
 
     if (defined $u->{passwordFile}) {