* lisp/xml.el: Handle entity and character reference expansion correctly.

* lisp/xml.el: Handle entity and character reference expansion correctly.
(xml-default-ns): New variable.
(xml-entity-alist): Use XML spec definitions for lt and amp.
(xml-parse-region): Make first two arguments optional.  Discard
text properties.
(xml-parse-tag-1): New function, spun off from xml-parse-tag.  All
callers changed.
(xml-parse-tag): Call xml-parse-tag-1.  For backward
compatibility, this function should not modify buffer contents.
(xml-parse-tag-1): Fix opening-tag regexp.
(xml-parse-string): Rewrite, handling entity and character
references properly.
(xml--entity-replacement-text): Signal an error if a parameter
entity is undefined.

* test/automated/xml-parse-tests.el (xml-parse-tests--data): More
testcases.