Doing a cpio from /var/empty if dir was not specified or non-existent
copies its mode to the destination. This is not desirable.
Rephrase this code to be more sensible.

PR:		173483
MFC after:	1 week
Remove cast that was only required for K&R C.

Reviewed by:	jilles
Replace a silly command sequence with a proper if-then-else.
Generate images sparsely. This saves space and time, especially when
generating images inside a VM (PR 173482).
Add a 'true' statement to last_orders to prevent some version of sh from
tripping over an empty function.