* lisp/nntp.el (nntp-request-post): Put a Message-ID generated by server if it

* lisp/nntp.el (nntp-request-post): Put a Message-ID generated by server if it
does not exist in the article.
(nntp-request-post): Run `nntp-prepare-post-hook'.
(nntp-wait-for): Save a response from the server in `nntp-process-response'
after sending a command.
(nntp-prepare-post-hook): New hook, run just before postting an article.
* lisp/nnheader.el (nnheader-init-server-buffer): Make `nntp-process-response'
be buffer-local in `nntp-server-buffer'.
(nntp-process-response): New variable, used for holding a response from the
server after sending a command.