- treat padding values passed to the "link" function as expressions to be
      evaluated rather than simply as literal values (part of #767372)
    - add a "default" function for trying to evaluate one expression, then
      another, then another... (part of #767372)
    - when creating a compat entry based on a real entry, set an entryUSN based on
      the source entry or the rootDSE (freeipa #864); the "scaffolding" entries
      won't have them