Improvements for old buffer compatibility

Now tree.h exports LIBXML2_NEW_BUFFER macro indicating that the
API uses the new buffers, important to keep code working with
both versions.
* tree.h buf.h: also export xmlBufContent(), xmlBufEnd(), and xmlBufUse()
          to help port the old code
* buf.c: make sure the compatibility counters are updated on
          buffer usage, to keep proper working of application compiled
	  against the old structures, but take care of int overflow