[FEATURE] Access module "leave unchanged"

Allow the access module to leave a selector
as "unchanged" in order to only overwrite
permissions and not the owner/group.

Resolves: #15619
Releases: master
Change-Id: If343565f34f9909d968c54b142a0c7d12108ac18
Reviewed-on: http://review.typo3.org/35491
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>