Rework vuxml a bit to make them validable again

modify tidy.xsl to make it generates manually the xml declaration
xsl is not able to generate a list of entity otherwise.

Remove copyright form included files, they are redudundant anyway and
in the end only the vuln.xml file is distribued with entities expanded

Rework a bit the entity declaration in order for the document to look
great after expansion (as it did before we introduced the expansion
mechanism)

All validation are now processed direcly on the flattened file.

This is based on a patch from mfechner here

Submitted by: mfechner
Differential Revision: https://reviews.freebsd.org/D28299