Added support for --catalogs to xmllint and xsltproc

[Daniel Veillard <veillard redhat com>]

  I commited this to libxslt and gnome-xml CVS heads.
  what it does is the following:

    grab the value of the environment SGML_CATALOG_FILES (should
probably be switched to XML_CATALOG_FILES at some point), and
load the contents of the catalogs pointed by this (it collects
PUBLIC SYSTEM, uses BASE and recurse on CATALOG c.f. [1]). this is
done at initialization time.

Then when an external entity is to be fetched the libxml entity lookup
default routine will lookup the associated table and fetch from this
file instead if an entry is found.

Basically, in an perfect world (unlike Red Hat 7.1 <grin/>) /etc/sgml/catalogs
could be used to avoid fetching the DTD and various entities declarations
needed to process DocBook from the net if the adequate set of DTD are
installed locally and registered in the catalogs.

Daniel

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