bug#26533: 26.0.50; xml-parse-region's symbol-qname argument is ignored

[Christopher Wellons]

A bug was introduced in aea67018 that causes the special "symbol-qnames"
value for PARSE-NS to be ignored, as if it were nil. This information is
discarded by the change to xml-parse-attlist, so functions further down
the line see the argument as if it was set to nil.

Here's an example of the bug:

    (with-temp-buffer
      (insert "<root a:b='c'></root>")
      (let ((xml-default-ns ()))
        (xml-parse-region nil nil nil nil 'symbol-qnames)))

Prior to this commit (Emacs 25.1 and earlier) the result is:

    ((root ((b . "c"))))

After this commit:

    ((root ((a:b . "c"))))

This is the same as PARSE-NS being set to nil.