Enhancement: Add `migrate` cmd to migrate from `s3legacy` to `default` layout

The `migrate` command for changing the `s3legacy` layout to the `default` layout
for s3 backends has been improved: It can now be restarted with `restic migrate
--force s3_layout` and automatically retries operations on error.

https://github.com/restic/restic/issues/1073
https://github.com/restic/restic/pull/1075
