Bugfix: Mark repository files as read-only when using the local backend

Files stored in a local repository were marked as writable on the
filesystem for non-Windows systems, which did not prevent accidental file
modifications outside of restic. In addition, the local backend did not work
with certain filesystems and network mounts which do not permit modifications
of file permissions.

restic now marks files stored in a local repository as read-only on the
filesystem on non-Windows systems. The error handling is improved to support
more filesystems.

https://github.com/restic/restic/issues/1756
https://github.com/restic/restic/issues/2157
https://github.com/restic/restic/pull/2989
