Separate payload archive close and free

Separate payload archive close and free

- When writing, archive finalization can write further data into
  the payload (eg cpio trailer). We need to be able to fish the
  final archive size *after* closing it, otherwise archive size
  will be off by trailer size.
- Add new rpmcpioFree() function which simpy frees the archive struct,
  calling rpmcpioClose() in the process if it wasn't already done.
- This also simplifies the error code gymnastics in fsm: we need to
  free the resources whether error or not, which rpmcpioFree() allows
  us to do. And for writers, calling rpmcpioClose() only needs to
  be done if earlier parts were successfull, so we dont need to worry
  about masking a former error code when calling it.