Add the saxon:systemId extension

For https://bugzilla.gnome.org/show_bug.cgi?id=519926

Both Xalan and Saxon have a systemId function (in their respective
namespaces) to provide the URI of the file that is being transformed.
It would be nice to have that function in libxslt1.1 as well, because
then DocBook could use it[0].

[0] See line 250 et seq in
/usr/share/xml/docbook/stylesheet/nwalsh/common/stripns.xsl