Bugfix: Exotic filenames no longer break restic backup's status output

Restic `backup` shows the names of files that it is working on. In previous
versions of restic, those names were printed without first sanitizing them,
so that filenames containing newlines or terminal control characters could
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