Enhancement: Include repository ID in the filesystem name used by `mount`

The filesystem exposed by the `mount` command now includes the repository ID
in its name. The ID is printed when opening a repository or can be read with
`restic cat config`.

```
$ df ./test-mount/
Filesystem        1K-blocks  Used Available Use% Mounted on
restic:d3b07384d1         0     0         0    - /mnt/my-restic-repo
```

https://github.com/restic/restic/issues/4868
https://github.com/restic/restic/pull/5243
