TemplateHelper: misc. cleanup of __default__ feature

* Don't require symbols to be listed twice; they can be listed in
  either __export__ or __default__, but need not be in both.
* Check for reserved words in __default__ list
bcfg2-lint: better checks for TemplateHelper __default__ stuff

* Check to ensure that no symbols use reserved words, which are
  different for __default__ than for __export__
* Check to ensure that symbols aren't listed in both __default__ and
  __export__
doc: Added docs for TemplateHelper __default__ feature