Fix issue #6926562: Ensure all multi-user cache files are managed correctly

Now we correctly iterate through the different user cache dirs.

Also update documentation to describe the new cache pruning behavior,
and deprecate the file modes for making files world readable/writable
which we really don't want people using any more.

Change-Id: I3708df3ddc697b1f5c511143cce7cc40a5a3d0bd