Use sequence information in MULTIBUF

This has two effects:

• It enables MULTIBUF to distinguish between the end of a file on the tape, like e.g. the end of the archive or contents directory, from the physical end of the tape. If you disable this switch, MULTIBUF will ask you whether to continue with the next medium whenever it detects an end of file. So you have to explicitly tell it ``NO'' after it has read the contents directory or after having read all the archive!
• It writes some sequence information to the tape and thus prevents you from using your tapes in the wrong order.