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 data-export="" and data-noexport="" attribues with value-less
  attributes literally named "export" and "noexport"

* drops "export" from any dfn that has a dfn type attribute