Use short syntax for dfn types, "for", (no)export

This change:

* replaces any data-dfn-type=element, etc., attributes with ones
  literally named "element", etc.

* replaces any data-dfn-for attribute with one literally named "for"

* replaces any data-export="" and data-noexport="" attributes with
  value-less attributes named "export" and "noexport"