xhtml1: Use file:// scheme in XML catalog

[mp@…]

There seems to be a regression in libxml2 v2.9.2 regarding the handling of files referenced in the XML catalog's URI attribute. In particular, when the (absolute) file path lacks a "​file://" scheme, libxml2 will append one itself and subsequently use it as part of the file name for the locally cached file, which fails.

I have described the issue at ​https://mail.gnome.org/archives/xml/2014-December/msg00000.html.

Meanwhile, using ​file://-based URIs in the catalog seems to mitigate the problem (at least for my use cases) and also seems to be a good practice. For example, Debian/Ubuntu also do it that way.

[mp@…]

patch