util, refactor: Use GetPathArg to read "-conf" value

Also "includeconf" values been normalized.

src/qt/guiutil.cpp

@@ -431,7 +431,7 @@ void openDebugLogfile()
 
 bool openBitcoinConf()
 {
-    fs::path pathConfig = GetConfigFile(gArgs.GetArg("-conf", BITCOIN_CONF_FILENAME));
+    fs::path pathConfig = GetConfigFile(gArgs.GetPathArg("-conf", BITCOIN_CONF_FILENAME));
 
     /* Create the file */
     std::ofstream configFile{pathConfig, std::ios_base::app};