file_unix: Do not try to preserve ownership when permission is -1
requested to merge work/fusionfuture/BUG447779-do-not-perserve-ownership-when-mode-is-minus-one into master
When the permission mode is set to -1, it means the copy job doesn't want to touch the dest permissions, but the dest ownership is always touched, which results in root password being required when creating a new file from templates.
This commit makes the copy job ignore dest ownership when the permission mode is -1, so no authentication dialog will pop up when creating a new file from templates.