This patch cleans up pkg/util/mount/* and pkg/util/volume/* to always
use filepath.Join instead of path.Join. filepath.Join is preferred
because path.Join can have issues on Windows.
This change affects the way that secret, configmap, downwardAPI and projected
volumes (which all use the same underlying code) implement their data update
functionality.
* Instead of creating a subdirectory hierarchy that itself
will contain symlinks to each actual data file, create only
symlinks to items in the root of the volume, whether they
be files or directories.
* Rather than comparing the user-visible data directory
to see if an update is needed, compare with the current
version of the data directory.
* Fix data dir timestamp format year
* Create ..data symlink even when a data volume has no data so
consumers can have simplified update watch logic.
