Correct file content extraction in Python 3

The "//" operator is required for integer division.  Use of "/"
caused icat to be fed a float for an offset.  Further, stderr is
a byte string, not a string, so the error message was also failing
in Python3.

This change is compatible with Python 2 and 3.

Signed-off-by: Alex Nelson <ajnelson@cs.ucsc.edu>
fixed version checking
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>
idifference: Accept .dfxml
Annotate idifference change
Merge branch 'master' into for_nist
Tweak dfxml_tool to compare against Fiwalk output with idifference
Optionally add directories to walk output