* nnheader.el (nnheader-parse-naked-head): New function.
(nnheader-parse-head): Use the above function, in order to handle
continuation lines properly.
(nnheader-remove-body): New function.
(nnheader-remove-cr-followed-by-lf): New function.
(nnheader-ms-strip-cr): Use the above function.
* gnus-agent.el (gnus-agent-regenerate-group): Call `nnheader-remove-body';
use `nnheader-parse-naked-head' instead of `nnheader-parse-head'.
* gnus-cache.el (gnus-cache-possibly-enter-article): Ditto.
* gnus-msg.el (gnus-inews-yank-articles): Do not unfold continuation lines by
itself;
call `nnheader-parse-naked-head' instead of `nnheader-parse-head'.
* nndiary.el (nndiary-parse-head): Ditto.
* nnfolder.el (nnfolder-parse-head): Ditto.
* nnimap.el (nnimap-retrieve-headers-progress): Ditto.
* nnmaildir.el (nnmaildir--update-nov): Ditto.
* nnml.el (nnml-parse-head): Ditto.