[BUGFIX] Generate unique id for checkboxes in flexform sections

Use a checksum of the element's name as additional identifier to generate
an unique id for checkboxes in flexform sections.

Resolves: #89092
Releases: master, 9.5
Change-Id: I57075aba05c8de23cbdacfff3a05b63eb71499c9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61911


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Sascha Rademacher <sascha.rademacher+typo3@gmail.com>
Tested-by: Christian Eßl <indy.essl@gmail.com>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Julian Geils <j_geils@web.de>
Reviewed-by: Sascha Rademacher <sascha.rademacher+typo3@gmail.com>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Armin Vieweg <armin@v.ieweg.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>