in. Sigh. Took me too long to find this. On the other hand, there is
more debugging and more asserts in the code. Also a -d option to turn
on progressive levels of debugging.

Also changed the operation of imageunzip so that individual slice
writes (ie, to unzip the BSD or Linux partition instead of the entire
disk). Using the slice device (/dev/rda0s1) is actually a problem on
BSD since it snoops writes to where the disklabel should be and alters
the offsets. Even worse, on non BSD partitions it craps out completely
because there is no disk label at all. This is really a dumb
thing. So, I added code to read the MBR when the -s (slice) option is
given, and use the MBR to compute the offsets from the beginning of
the raw disk. Must always use the raw disk device now, and the new
operation is:

	imageunzip all.ndz /dev/rad0
	imageunzip -s 2 rhat.ndz /dev/rad0

Note that if you give a slice instead of a disk device, and there is a
valid looking MBR in the slice (which is very possible), then things
will get very confused. Not sure what to do about that yet.