Change: Do not save atime by default

By default, the access time for files and dirs is not saved any more. It is
not possible to reliably disable updating the access time during a backup,
so for the next backup the access time is different again. This means a lot
of metadata is saved. If you want to save the access time anyway, pass
`--with-atime` to the `backup` command.

https://github.com/restic/restic/pull/1452
