[BUGFIX] Files contents cannot be changed

Setting file contents failed for two reasons:

a) The permissions were incorrectly read from TSconfig and

b) We used a new "update" permission that was not present
   in the default configuration, so without manually setting it,
   a file could never be updated, even by admins.

This commit changes the "update" permission back to "edit", which was
also used in the pre-FAL era.

Change-Id: I408a19257a50868f7978ffac32eefbd410117e2d
Resolves: #37812
Resolves: #39860
Resolves: #39949
Releases: 6.0
Reviewed-on: http://review.typo3.org/14257
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel