[BUGFIX] Fix permissions of downloaded translations into l10n folder

The downloaded extension translation files should have proper permissions
set after having been unzipped into l10n directory.

Change-Id: I59169bc26cf3a03dc5d9379b5b496ef5b51fd05b
Fixes: #31964
Releases: 4.7, 4.6
Reviewed-on: http://review.typo3.org/6782
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Stefan Neufeind
Reviewed-by: Philipp Gampe
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter