mainly motivated by u2cli's folder syncing in turbo mode
which would un-turbo on most dupes due to wrong lastmod
disabled by default for regular http listings
(to avoid confusion in most regular usecases),
enable per-request with urlparam lt
enabled by default for single-level webdav listings
(because rclone hits the same issue as u2cli),
can be disabled with arg --dav-rt or volflag davrt
impossible to enable for recursive webdav listings
