Improve unused disk block zeroing (#725)

* Move optional steps for testing VMs out of README.md file
This may help avoid new users being confused that they must follow
those instructions.

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

* Add 2025-Jan-30 versions of source code for install.sh

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

* Improve the method used to zero out unused disk space in a VM

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

---------

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

Author: jafingerhut

vm-ubuntu-24.04/clean.sh

@@ -32,9 +32,9 @@ sudo apt clean
 # image files.
 
 echo "Writing zeros to unused disk blocks (be patient) ..."
-FNAME=`mktemp --tmpdir big-empty-zero-file-XXXXXXXX`
-dd if=/dev/zero of=${FNAME} bs=4096k
-/bin/rm -f ${FNAME}
+FNAME="/bigemptyfile"
+sudo dd if=/dev/zero | sudo dd of=${FNAME} bs=4096k
+sudo /bin/rm -f ${FNAME}
 
 echo "Disk usage before running this script:"
 echo "$DF1_BEFORE"