[BUGFIX] FormEngine: FlexForm section container not saved

Naming prefixes in flex form section container handling is changed
to successfully save them again.

Resolves: #66663
Releases: master
Change-Id: I8268a4cd9860c4b17a54e05ba2aa86190fa7eda2
Reviewed-on: http://review.typo3.org/39110


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>