Use with-current-buffer.

Use with-current-buffer.
(nntp-send-buffer): Just set the buffer to unibyte rather than use the
dubious mm-with-unibyte-current-buffer.
(nntp-with-open-group-function): New function extracted from
nntp-with-open-group macro.
(nntp-with-open-group): Use the function, so it's easier to debug.
Add indentation and debugging info.
(nntp-open-telnet-stream, nntp-open-via-rlogin-and-telnet): Recommend the
use of the netcat alternatives.