sgml-xml-mode is a user option that is over-riden by smgl-mode. When I
customize sgml-xml-mode to be enabled for every sgml-mode derived mode
(specifically html-mode), it is disabled because a HTML file does not
have to be a valid XML file -- even if I would want it to be.

This patch only sets sgml-xml-mode if we know that the current file is
an XML file, but leaves the default value if it cannot be said for sure.

-- 
	Philip K.