Added security fix of version 1.6.10

git-svn-id: https://svn.typo3.org/TYPO3v4/Extensions/powermail/branches/1.7.x@76376 735d13b6-9817-0410-8766-e36946ffe9aa
Added feature 37453: Provide backaction (Thanks to Jan Bednarik)

git-svn-id: https://svn.typo3.org/TYPO3v4/Extensions/powermail/branches/1.7.x@76380 735d13b6-9817-0410-8766-e36946ffe9aa