[BUGFIX] Clean MP parameter for rootline cache identifier

Any commas in the mount point parameter are replaced with underscores
to prevent caching errors in the RootlineUtility.

Resolves: #57073
Releases: 6.2
Change-Id: If1e135d0abbc08ebc11954d17f57ea037983ba10
Reviewed-on: https://review.typo3.org/28521
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Jost Baron
Tested-by: Jost Baron
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert