-N//--timestamping: Use xattr infos (if available)

When a download with -N is interrupted, the incomplete file won't have the correct timestamp. Then, when continuing with -N and --if-modified-since on (that is the default), we send this wrong (newer) timestamp and the download won't be continued.

To avoid this, either use --no-if-modified-since (to be documented !) or take advantage of the xattr feature (where we have to store the timestamp of the file at the moment we open it).