Bugfix: Sanitize filenames printed by `backup` during processing

The `backup` command would previously not sanitize the filenames it printed
during processing, potentially causing newlines or terminal control characters
to mangle the status output or even change the state of a terminal.

Filenames are now checked and quoted if they contain non-printable or
non-Unicode characters.

https://github.com/restic/restic/issues/2260
https://github.com/restic/restic/issues/4191
https://github.com/restic/restic/pull/4192