Commit schema that validates current Fiwalk output

The Fiwalk output validated comes from this commit:

    https://github.com/ajnelson/sleuthkit.git
    603737e9a86b53f9d843beef47fc5860789c7117

Validation command (and exemplar data):

    xmllint --schema dfxml.xsd m57-redacted-charlie-2009-12-08.aff.dfxml >/dev/null

Some documentation cleanup remains.