[users] Handle detailed groups list

Groups can be specified with must_exist and/or system set,
so they fill in the groups list more carefully.

src/modules/users/Config.cpp

@@ -605,10 +605,13 @@ setConfigurationDefaultGroups( const QVariantMap& map, QList< GroupDescription >
         }
         else
         {
+            // By default give the user a handful of "traditional" groups, if
+            // none are specified at all. These are system (GID < 1000) groups.
             cWarning() << "Using fallback groups. Please check *defaultGroups* value in users.conf";
             for ( const auto& s : { "lp", "video", "network", "storage", "wheel", "audio" } )
             {
-                defaultGroups.append( GroupDescription( s, GroupDescription::CreateIfNeeded{}, GroupDescription::SystemGroup{} ) );
+                defaultGroups.append(
+                    GroupDescription( s, GroupDescription::CreateIfNeeded {}, GroupDescription::SystemGroup {} ) );
             }
         }
     }
@@ -620,6 +623,21 @@ setConfigurationDefaultGroups( const QVariantMap& map, QList< GroupDescription >
             {
                 defaultGroups.append( GroupDescription( v.toString() ) );
             }
+            else if ( v.type() == QVariant::Map )
+            {
+                const auto innermap = v.toMap();
+                QString name = CalamaresUtils::getString( innermap, "name" );
+                if ( !name.isEmpty() )
+                {
+                    defaultGroups.append( GroupDescription( name,
+                                                            CalamaresUtils::getBool( innermap, "must_exist", false ),
+                                                            CalamaresUtils::getBool( innermap, "system", false ) ) );
+                }
+                else
+                {
+                    cWarning() << "Ignoring *defaultGroups* entry without a name" << v;
+                }
+            }
             else
             {
                 cWarning() << "Unknown *defaultGroups* entry" << v;

src/modules/users/Config.h

@@ -41,10 +41,18 @@ class GroupDescription
 {
 public:
     // TODO: still too-weakly typed, add a macro to define strongly-typed bools
-    class MustExist : public std::true_type {};
+    class MustExist : public std::true_type
-    class CreateIfNeeded : public std::false_type {};
+    {
-    class SystemGroup : public std::true_type {};
+    };
-    class UserGroup : public std::false_type {};
+    class CreateIfNeeded : public std::false_type
+    {
+    };
+    class SystemGroup : public std::true_type
+    {
+    };
+    class UserGroup : public std::false_type
+    {
+    };
 
     ///@brief An invalid, empty group
     GroupDescription() {}