[BUGFIX] Form: Confirmation value relies on label value

The confirmation action (back to form or send mail) relies
only on the label value. To solve this, the confirmation-true
and confirmation-false arguments are introduced.

Change-Id: I6dbbb881396ccb1c215a89e7237766115dd03717
Fixes: #31560
Releases: 4.6, 4.7
Reviewed-on: http://review.typo3.org/6530
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader