[BUGFIX] meaningfulTempFilePrefix is not multi-byte safe

On a utf-8 file system meaningful prefixes can contain utf-8 characters. To
prevent truncating in the middle of a multi-byte character the API functions
must be used instead of substr().

Change-Id: I760f80b8cfa2dcea8b554c24467f56b662d465d9
Resolves: #30416
Releases: 4.5, 4.6
Author: Steffen Gebert <steffen.gebert@typo3.org>
Reviewed-on: http://review.typo3.org/5479
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert