Minor updates to iredact.py and iredact-config.txt

Updated broken xml_file and image_file references in close_files.
Cleaned up iredact-config.txt.

AJN remark: These changes are necessary, but not yet sufficient, to
restore iredact.py to a working state.  The configuration documentation
is improved.

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