tmpfiles: create /var/{lib,log,cache}/private during early boot

tmpfiles: create /var/{lib,log,cache}/private during early boot

This directory is used by the DynamicUer= stuff when used in combination
with StateDirectory=/LogDirectory=/CacheDirectory=. Let's make sure the
dir exists early on with the right perms. This is not strictly necessary
as we'll also create the dir on demand if it is missing, but in the
interest of grabbing the name early on, and making things more explicit
let's also list this in a tmpfiles.d/ snippet.