company-template-c-like-templatify: use "argN" as actual field text

* company-template-add-field: Change the third argument type back; add an
  optional fourth argument; move point to the end of the field.

* company-template-c-like-templatify, company-clang-objc-templatify: Adjust for
  the above. Use `point-marker'.

* company-template-field-at: Extract from `company-template-forward-field'.