Allow gnus backends to return a list of headers
* lisp/gnus/nnselect.el (nnselect-add-novitem): New macro. (nnselect-retrieve-headers): Allow backends to return headers as a list. Return this list rather than re-populating the server buffer with 'nov format headers. * lisp/gnus/gnus-sum.el (gnus-fetch-headers): Allow backends to return headers as a list. Make sure the dependencies structure is updated.