Win32: reworked ngx_win32_rename_file() to check errors.

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.