Improve `HTTPFileSystem::Stat`

[bbockelm]

The HTTPFileSystem::Stat code currently just invokes HTTPFile::Stat. This works -- provided the object in question isn't a directory!

Let's make the HTTPFileSystem code a bit more flexible and handle either directories or files. If we're talking to a WebDAV backend (see #72), then use the PROPFIND verb instead and look for the collection attribute.

As with the directory listing, the xrdcl-pelican code has XML parsing code worth reusing (or just using as inspiration).