Browser upload doesn't work with multi-web server architecture

On sites with multiple web servers, uploads using the browser upload feature fail due to unshared temporary directories, which is a common set up.

The cause of this is file_save_upload is passed without a $dest parameter:

function _user_import_file($import_id = NULL, $ftp_file_selected = NULL) {

    static $file;
    if (!empty($file)) return $file;

    // file was uploaded through browser
    $file = file_save_upload('file_upload');
    if (!empty($file) ) return $file;