Fix various issues with url-dav package (Bug#11916).

Fix various issues with url-dav package (Bug#11916).

* url-dav.el (url-dav-supported-p): Added doc-string and remove
check for feature `xml' and function `xml-expand-namespace' which
never existed in Emacs proper.
(url-dav-process-response): Remove all indentation and newlines
from XML before parsing.  Change call to `xml-parse-region' to do
namespace expansion with simple qualified names (Bug#11916).
(url-dav-request): Add autoload.
(url-dav-directory-files): Properly deal with empty directories.
Unhex URL before generating relative URLs.
(url-dav-file-directory-p): Fix check for 'DAV:collection.