(nntp-insert-buffer-substring, nntp-copy-to-buffer): New macros.

(nntp-insert-buffer-substring, nntp-copy-to-buffer): New macros.
(nntp-wait-for, nntp-retrieve-articles, nntp-async-trigger)
(nntp-retrieve-headers-with-xover): Use nntp-insert-buffer-substring to copy
 data from unibyte buffer to multibyte current buffer.
(nntp-retrieve-headers, nntp-retrieve-groups); Use nntp-copy-to-buffer to copy
 data from unibyte current buffer to multibyte buffer.
(nntp-make-process-buffer): Make process buffer unibyte.