* xml.el: Implement XML parameter entities.

* xml.el: Implement XML parameter entities.
(xml-parameter-entity-alist): New variable.
(xml-parse-region, xml-parse-fragment): Preserve previous values
of xml-entity-alist and xml-parameter-entity-alist, so that
repeated calls on different documents do not change them.
(xml-parse-tag): Fix doctype regexp.
(xml--entity-replacement-text): New function.
(xml-parse-dtd): Use it.  Don't handle system entities; doing that
properly requires url retrieval which is unimplemented.
(xml-escape-string): Doc fix.