Add libxml extension for common code that is needed to be shared between

Add libxml extension for common code that is needed to be shared between
various xml extensions.  currently the only implemented support is the
addition of the streams support for libxml.  One new function,
libxml_set_streams_context, which allows a streams context to bet set
prior to loading or writing documents.  This works transparently with any
extension that uses libxml.  All ini settings that effect streams will
also now effect the loading and writing of xml documents.

TODO: linux support, not sure if config.m4 will work right.