Enhancement: mount: Enforce FUSE Unix permissions with allow-other

The fuse mount (`restic mount`) now lets the kernel check the permissions of
the files within snapshots (this is done through the `DefaultPermissions` FUSE
option) when the option `--allow-other` is specified.

To restore the old behavior, we've added the `--no-default-permissions` option.
This allows all users that have access to the mount point to access all
files within the snapshots.

https://github.com/restic/restic/pull/2017
