Revise DFXML output to validate against pending schema

If this patch is accepted, Hashdeep's DFXML output will validate
against the DFXML schema as in this branch:
<https://github.com/ajnelson/dfxml_schema/tree/archive/for_hashdeep_v0>

The changes from that schema branch will be merged into the upstream
DFXML schema after a request-for-comments period (assuming no
objections).

This patch has not been tested beyond these commands:

    git clone --branch=validate_dfxml https://github.com/ajnelson/hashdeep.git
    git clone --branch=archive/for_hashdeep_v0 https://github.com/ajnelson/dfxml_schema.git
    cd hashdeep
    sh bootstrap.sh && ./configure --prefix=$PWD/build && make -j && make install
    build/bin/hashdeep -d -r . >test_hashdeep.xml
    xmllint --noout --schema ../dfxml_schema/dfxml.xsd test_hashdeep.xml

The output of the last command was:

    test_hashdeep.xml validates