Enhancement: support Alibaba/Aliyun OSS with S3 backend

We've added a new flag to the s3 backend `s3.bucket-lookup` which can
be set to these 3 values:

- `auto` - existing behaviour
- `dns` - use DNS style bucket access
- `path` - use path style bucket access

To make the s3 backend work with Alibaba/Aliyun OSS you must set
`s3.bucket-lookup` to `dns` and set the `s3.region` parameter.  For
example:

    restic -o s3.bucket-lookup=dns -o s3.region=oss-eu-west-1 -r s3:https://oss-eu-west-1.aliyuncs.com/bucketname init

Note that s3.region must be set otherwise the minio SDK tries to look
it up and it seems that Alibaba doesn't support that properly.

https://github.com/restic/restic/issues/2528
https://github.com/restic/restic/pull/2535