Let s3cmd have the correct bucket name when using s3 scheme

I was trying out an s3-compatible service, and needed to give their hostname instead of using the default aws s3 hostname.  So I set DEST to s3://host/bucket/ .  This worked fine for backup and restore.  The only part that had a problem was the part that gets the remote backup size.  This commit makes that work when using the s3:// scheme.