Enhancement: Add Alibaba/Aliyun OSS support in the `s3` backend

A new extended option `s3.bucket-lookup` has been added to support
Alibaba/Aliyun OSS in the `s3` backend. The option can be set to one
of the following 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
