dfxml_tool: Add path prefix removal option

dfxml_tool.py optionally includes the filename of each file it
encounters during a walk.  However, the path specification is included,
though it may not be desired at times.  Now, the following command
produces output with files name-comparable to Fiwalk:

    python dfxml_tool.py --stripprefix="./" .

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>