Allow custom display-buffer actions in ERC

Allow custom display-buffer actions in ERC

* doc/misc/erc.texi: Add new section under "Integrations" chapter
describing `display-buffer' Custom function choice for ERC's many
buffer-display options.
* etc/ERC-NEWS: Mention new function variant for all buffer-display
options.
* lisp/erc/erc-backend.el: Add forward declaration for
`erc--called-as-input-p' and `erc--display-context'.
(erc--server-reconnect-display-timer,
erc--server-last-reconnect-display-reset): Use new name for option
`erc-reconnect-display', now `erc-auto-reconnect-display'.
(erc--server-determine-join-display-context): New generic function to
determine value of `erc--display-context' during JOINs.
(erc-server-JOIN, erc-server-PRIVMSG): Set `erc--display-context' to a
symbol for the handler's IRC command, like `JOIN', for the benefit of
custom `display-buffer'-like functions running in `erc-setup-buffer'.
(erc-server-471, erc-server-471-functions, erc-server-473,
erc-server-473-functions): New handlers for JOIN rejections.  Also
remove 471 and 473 from comment at bottom of file.
(erc-server-475): Bind `erc--called-as-input-p' so that `erc-cmd-JOIN'
sets `erc-interactive-display' context.
* lisp/erc/erc-join.el (erc-autojoin-mode, erc-autojoin-enable,
erc-autojoin-disable): Kill local variable
`erc-join--requested-channels'.  Add and remove
`erc-join--remove-requested-channels' to/from various server-handler
hooks for JOIN rejection numerics.
(erc-join--requested-channels): New local variable to remember
channels we've attempted to JOIN this session that haven't yet been
confirmed by the server.
(erc-join--remove-requested-channel): New JOIN rejection handler to
stop tracking channel in `erc-join--requested-channels'.
(erc--server-determine-join-display-context): module-specific
implementation of generic function for `erc-autojoin-mode'.
(erc-autojoin--join): Remember channels slated for JOIN'ing.
* lisp/erc/erc.el (erc--buffer-display-choices): New helper constant
for defining common `:type' for all buffer-display options.
(erc-buffer-display, erc-interactive-display,
erc-auto-reconnect-display, erc-receive-query-display): Use helper
`erc--buffer-display-choices' for defining `:type', which
includes a new choice for a `display-buffer'-like function.
(erc-reconnect-display, erc-auto-reconnect-display): Alias former to
latter, now the preferred name.
(erc-reconnect-timeout, erc-auto-reconnect-timeout): Change name from
former to latter.  This option is new in ERC 5.6.
(erc-reconnect-display-server-buffers): New option.
(erc-buffer-do): Revise doc string.
(erc--display-context): New variable, an alist of "context tokens" to
be forwarded as the "action alist" to `erc-buffer-display' functions.
(erc-skip-displaying-selected-window-buffer): New variable, deprecated
at birth, to act as an escape hatch for folks who don't want to skip
the displaying of buffers already showing in the selected window.
(erc--display-buffer-overriding-action): Local variable allowing
modules to influence the displaying of new ERC buffers independently
of user options.
(erc-setup-buffer): Do nothing when the selected window already shows
current buffer unless user has provided a custom display function.
Accommodate new Custom choice function values, like `display-buffer'
and `pop-to-buffer'.
(erc-open): Run `erc-setup-buffer' when option
`erc-reconnect-display-server-buffers' is non-nil, even for existing
server buffers.  Bind `display-buffer-overriding-action' to the value
of `erc--display-buffer-overriding-action' around calls to
`erc-setup-buffer'.
(erc-select-read-args): Add `erc--display-context' to environment.
(erc, erc-tls): Bind `erc--display-context' around calls to
`erc-select-read-args' and main body.
(erc-cmd-JOIN, erc-cmd-QUERY, erc--cmd-reconnect, erc-handle-irc-url):
Add item for `erc-interactive-display' to `erc--display-context'.
(erc-connection-established): Update name of
`erc-reconnect-display-timeout' to
`erc-auto-reconnect-display-timeout'.
(erc-message-english-s471, erc-message-english-s473): New variables,
format templates for JOIN rejection messages.
* test/lisp/erc/erc-scenarios-base-buffer-display.el
(erc-scenarios-base-buffer-display--defwin-recbury-intbuf,
erc-scenarios-base-buffer-display--defwino-recbury-intbuf,
erc-scenarios-base-buffer-display--count-reset-timeout): Use preferred
name `erc-auto-reconnect-display' for `erc-reconnect-display'.
* test/lisp/erc/erc-scenarios-join-display-context.el: New file.
* test/lisp/erc/erc-tests.el (erc--initialize-markers): Fix
unrealistic call to `erc-open'.
(erc-setup-buffer--custom-action): New test.
(erc-select-read-args, erc-tls, erc--interactive, erc-server-select):
Expect new environment binding for `erc--display-context'.
* test/lisp/erc/resources/join/buffer-display/mode-context.eld: New
file.  (Bug#62833)