[xml] _private and userdata for nodes

["Hundiak, Arthur" <ahundiak ingr com>]

Is it safe to assume that the _private variable in the xmlAttr and xmlNode
structures can be used for application specific data?

I couldn't find anything specific in the documentation.  Gdome uses this
pointer for user data.  There was also a discussion back in March concerning
tree callbacks which referred to _private as an application reserved
pointer. Xslt makes some use of these variables.

Maybe the comments in tree.h could be changed from "for Cobra, Must be
first" to "application data".