When uploading chunked files to nextcloud, it gives a 423 error while
it is merging files.
This waits for an exponentially increasing amount of time for it to
clear.
If after we have received a 423 error we receive a 404 error then we
assume all is good as this is what appears to happen in practice.
Fixes#7109
Fix https://github.com/rclone/rclone/issues/7103
Before this change the RegExp validating the endpoint URL was a bit
too strict allowing only /dav/files/USER due to chunking limitations.
This patch adds back support for /dav/files/USER/dir/subdir etc.
Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>