Enhancement: Add `--delete` option to `restore` command

The `restore` command now supports a `--delete` option that allows removing
files and directories from the target directory that do not exist in the
snapshot. This option also allows files in the snapshot to replace non-empty
directories having the same name.

To check that only expected files are deleted, add the `--dry-run --verbose=2`
options.

https://github.com/restic/restic/issues/2348
https://github.com/restic/restic/pull/4881
