libsync: use the new webdav url if the server reports it

libsync: use the new webdav url if the server reports it

The rules to select the webdav url are now:

 - If the server reports that the new chunking algorithm is working,
   always use remote.php/dav/files/<username>
   This capability can be overriden with an environment variable

 - Otherwise, use the dav path provided by the theme, which defaults to
   remote.php/webdav

This means that with the newer server, the branding can no longer override
the webdav URL. If there is still an usecase for the branding to do so, we
need to find another way to override it. But it is now more complicated to
configure as might need include the username and need different endpoint
depending on the operations (chunks or not)

Issue #4007