Enhancement: Make overwrite behavior of `restore` customizable

The `restore` command now supports an `--overwrite` option to configure whether
already existing files are overwritten. The default is `--overwrite always`,
which overwrites existing files. `--overwrite if-newer` only restores files
from the snapshot that are newer than the local state. And `--overwrite never`
does not modify existing files.

https://github.com/restic/restic/issues/4817
https://github.com/restic/restic/issues/200
https://github.com/restic/restic/pull/4837