Change: Deprecate `check --check-unused` and add further checks

Since restic 0.12.0, it is expected to still have unused blobs after running
`prune`. This made the `--check-unused` option of the `check` command rather
useless and tended to confuse users. This option has been deprecated and is
now ignored.

The `check` command now also warns if a repository is using either the legacy
S3 layout or mixed pack files with both tree and data blobs. The latter is
known to cause performance problems.

https://github.com/restic/restic/issues/3295
https://github.com/restic/restic/pull/3730
