it "-" for the output filename. This means we cannot seek, and so the
little bit of stuff we do for netdisk cannot be done, but so what.
It also means that output errors are fatal, and so no reason to retry
them (like we do for writes to an NFS file).

Convert imageunzip to optionally using stdin instead of a file, by
giving it "-" for the input name. I added an 1MB chunk buffer to,
which reads are buffered into. I also changed unified the interface
from the the Frisbee client, which got rid of some ifdefs.