Bugfix: Mount command should return exit code 0 after receiving Ctrl-C

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

We have changed the exit code to zero as this is the expected way to stop the
mount command.

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