Don't kill server buffer with erc-kill-buffer-on-part

Don't kill server buffer with erc-kill-buffer-on-part

* etc/ERC-NEWS: Mention new flag `erc-killing-buffer-on-part-p' and
the renaming of `erc-kill-channel'.
* lisp/erc/erc-backend.el (erc-server-PART): Only kill a buffer on
behalf of `erc-kill-buffer-on-part' when the buffer hasn't already
been killed, and bind `erc-killing-buffer-on-part-p' to t when doing
so.
* lisp/erc/erc-log.el (erc-conditional-save-buffer): Don't save logs
when the buffer parameter is nil because that causes the server buffer
to be saved out.  It's possible that user code relying on this
longstanding bug will be affected, however, by default, the server
buffer will also be saved out independently at designated junctures.
* lisp/erc/erc.el (erc-part-hook): Redo doc string.
(erc-killing-buffer-on-part-p): New variable, a flag to prevent
redundant execution of `erc-kill-channel-hook' members concerned with
parted channels.
(erc-kill-buffer-on-part): Tweak doc string.
(erc-kill-channel-hook): Use new name for `erc-kill-channel',
`erc-part-channel-on-kill'.
(erc-kill-channel, erc-part-channel-on-kill): Rename former to latter,
and inhibit execution when `erc-killing-buffer-on-part-p' is non-nil.
* test/lisp/erc/erc-scenarios-base-kill-on-part.el: New file.
(Bug#70840)