unbreak XDG_CONFIG_DIRS and by extension autostart

"If $XDG_CONFIG_DIRS is either not set or empty, a value equal to
/etc/xdg should be used."

If XDG_CONFIG_DIRS was not set prior to startplasma, which it ordinarily
would not be, then the previous code would set the value to "$newdir:"
which was silly and also wrong as per the spec quoted above. Instead
default to /etc/xdg/.

Also camelCase the variables to follow our coding style.

startkde/startplasma.cpp

@@ -330,9 +330,12 @@ void setupPlasmaEnvironment()
     qputenv("KDE_APPLICATIONS_AS_SCOPE", "1");
 
     // Add kdedefaults dir to allow config defaults overriding from a writable location
-    const auto current_xdg_config_dirs = qgetenv("XDG_CONFIG_DIRS");
-    const auto new_xdg_dir = QStandardPaths::writableLocation(QStandardPaths::GenericConfigLocation).toUtf8() + "/kdedefaults";
-    qputenv("XDG_CONFIG_DIRS", new_xdg_dir + ":" + current_xdg_config_dirs);
+    QByteArray currentConfigDirs = qgetenv("XDG_CONFIG_DIRS");
+    if (currentConfigDirs.isEmpty()) {
+        currentConfigDirs = "/etc/xdg";
+    }
+    const auto extraConfigDir = QStandardPaths::writableLocation(QStandardPaths::GenericConfigLocation).toUtf8() + "/kdedefaults";
+    qputenv("XDG_CONFIG_DIRS", extraConfigDir + ":" + currentConfigDirs);
 }
 
 void setupX11()