Bugfix: Make `mount` return exit code 0 after receiving Ctrl-C / SIGINT

To stop the `mount` command, a user has to press Ctrl-C or send a SIGINT
signal to restic. This used to cause restic to exit with a non-zero exit code.

The exit code has now been changed to zero as the above is the expected way
to stop the `mount` command and should therefore be considered successful.

https://github.com/restic/restic/issues/2015
https://github.com/restic/restic/pull/3894
