[SECURITY] Information Disclosure in the Configuration Module

The configuration module showed the encryption key as plaintext.
For this view, the encryption key is masked and it's length is
shown instead, e.g. "***** (length: 96 characters)"

Change-Id: I16145e76a60d15d8e9575ef0cc5cf3cd54b1b6b1
Fixes: #39345
Releases: 6.0, 4.7, 4.6, 4.5
Security-Commit: c9b4932c07d1b95c47e5c184b74c2d3493db3b06
Security-Bulletin: TYPO3-CORE-SA-2012-004
Reviewed-on: http://review.typo3.org/13768
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader