fixes #2436 refactor async upload authentication

Replaces direct login check with ws_session_login for 'pwg.images.uploadAsync' and updates error handling to return the login response. Also updates the web service method definition to make username and password optional and clarifies admin credential requirements in the documentation.

include/user.inc.php

@@ -102,10 +102,19 @@ if (
   and isset($_POST['password'])
 )
 {
-  if (!try_log_user($_POST['username'], $_POST['password'], false))
+  include_once(PHPWG_ROOT_PATH.'include/ws_init.inc.php');
+  include_once(PHPWG_ROOT_PATH.'include/ws_functions/pwg.php');
+
+  $credentials = array(
+    'username' => $_POST['username'],
+    'password' => $_POST['password']
+  );
+
+  $login = ws_session_login($credentials, $service);
+  
+  if (true !== $login)
   {
-    include_once(PHPWG_ROOT_PATH.'include/ws_init.inc.php');
-    $service->sendResponse(new PwgError(999, 'Invalid username/password'));
+    $service->sendResponse($login);
     exit();
   }
   $_SESSION['connected_with'] = 'pwg.images.uploadAsync';