Log Message:
Preserve file permissions when copying files from the ISO to the
memory stick image, because otherwise recent NetBSD versions that use
root on cd9660 instead of a ramdisk won't boot.

This is currently implemented by using a vnd mount instead of isoinfo -x
to extract the ISO contents.  This solution is less than ideal since
it requires root permissions and will fail if vnd0 is already in use.
Still, it's better than not working at all.  OK'd by agc.