Win32: reworked ngx_win32_rename_file() to check errors. Previously, ngx_win32_rename_file() retried on all errors returned by MoveFile() to a temporary name. It only make sense, however, to retry when the destination file already exists, similarly to the condition when ngx_win32_rename_file() is called. Retrying on other errors is meaningless and might result in an infinite loop.