[unit] unit context "overwrite" option

This option is used to tell that on restoration data should be overwritten but
not updated

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[unit] use hardlinks to speed up backup process

There is no check yet for backup target is in the same filesystem

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>