[TASK] Rework t3lib_utility_Mail::breakLinesForEmail to drop

 strrev-magic

Use strrpos/substr instead of strrev-magic.
Add testcase for one more cornercase when breaking lines.

Change-Id: Icfa5bb44dc750100ed97aa4e22f43263d2072d26
Resolves: #28039
Releases: 4.7
Reviewed-on: http://review.typo3.org/5133
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter