[xml] Support for really large XML documents

[Vit Zikmund <vit_zikmund cz ibm com>]

Greetings libxml gurus!
We are using XMLSec library built on
top of libxml2 to process some large XML files, however it doesn't seem
to work for files >2GB, which is unfortunately what we need.

I'd like to ask if the library should
support processing that large files (otherwise, this might be a bug).

It seems there's a limitation in the
struct
_xmlOutputBuffer, that stores
written bytes in a signed int - therefore the max limit is 2GB.
Here it is: http://git.gnome.org/browse/libxml2/tree/include/libxml/xmlIO.h#n141

We'd really like if the library could
support 64 bit sizes and I see the struct
_xmlParserInputBuffer, that's
nearby, does. It uses unsigned long that's 64bit for x86_64 architecture,
we are building for.
It might really help us if someone here
could know what else will need to be fixed for the whole thing to work.
If it's going to be a patch or a full scale project.

Thanks in advance for your help.
Vit Zikmund