[FEATURE] Introduce read-only column for file mounts

File mount records get a new flag "read only".
This flag replaces the virtual flag introduced earlier,
so it can be defined natively in the record.

Resolves: #59830
Releases: 6.3
Change-Id: I11c738cb4740ea20b9088cc796ea0fb7ced1107a
Reviewed-on: http://review.typo3.org/31096
Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Bernhard Kraft <kraftb@think-open.at>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>