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

* 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-async-trigger): Save a response from the server in
`nntp-process-response' after sending a command.
(nntp-wait-for): Ditto.
(nntp-prepare-post-hook): New hook, run just before postting an article.
* 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.