support S3 buckets with custom CNAME and dot in bucket name

S3 buckets with a dot in the bucket name must call the path-style endpoint (s3-{region}.amazonaws.com/{bucketname}) insteal of the virtual host-style endpoint ({bucketname}.s3.amazonaws.com). This support is required to enable custom CNAMEs to function correctly (e.g. for `cdn.mydomain.com CNAME cdn.mydomain.com.s3.amazonaws.com` where the publicly seen domain is cdn.mydomain.com).