bug#11916: 24.1.50; Making url-dav work

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> Attached. I had to go another route, though; turns out the `parse-ns'
> argument is already overloaded in `xml-parse-tag' (it can be used to
> provide a namespace->URI mapping), but that wasn't mentioned in the
> other parse functions. So I had to introduce an additional argument.

I'd seen that, indeed, but I think that since this parse-ns arg is
mostly passed around and only finally used in one place, I'd rather not
add an argument but instead pass both values via the single
parse-ns argument.  parse-ns could then be:
- nil as before.
- an alist of namespace->URI.
- a cons cell (symbol-qnames . ALIST) which does the same as the
  previous one but uses symbols instead of cons cells for qnames.
- the symbol `symbol-qnames' to mean (symbol-qnames . STANDARD-ALIST).
- t to mean STANDARD-ALIST.
The last two are only allowed when entering xml-parse-region but not in
recursive calls (and not in calls to xml-maybe-do-ns).


        Stefan