Split vuln.xml file [2/2]

The vuln.xml file has grown a lot since 2003. To avoid having to unlock
the svn size limitation, the file is now split into 1 file per year up
to the current year + previous one. The split is made based on the date
when the entry has been added.

In order to achieve the split without breaking any consumer we use a standard
XML mechanism via the definition of entities.

While here add a new target make vuln-flat.xml which will expand the entities
in order to be able to regenerate a one uniq file if needed. This useful to for
example allow to test with pkg audit directly given the XML parser used in pkg
does not support custom entities.

The vuxml web site generator has been modified to ensure the vuln.xml file it
provides is the expanded version, so for consumers it is still only one single
file to download.