Extend erc-interactive-display to cover /JOINs

Extend erc-interactive-display to cover /JOINs

* lisp/erc/erc.el (erc-display): Mention that buffer-related display
options live in the customization group `erc-buffers'.
(erc-buffer-display, erc-join-buffer): Swap alias and aliased so that
the favored name, `erc-buffer-display', appears in the definition and
in the Customize menu.  Also note related buffer-display options in
the doc string.
(erc-query-display, erc-interactive-display): Make the former an alias
of the latter, new in ERC 5.6, because their roles were functionally
redundant and thus confusing.  Inherit the default value from
`erc-query-display' because users are more familiar with the pop-up
window behavior than a single-window replacement.
(erc-reconnect-display): Use preferred name for cross-referencing
fallback option `erc-buffer-display' in doc string, and explain how
/reconnect handling differs.
(erc--setup-buffer-hook): Add new internal hook for modules that
operate on windows and frames, such as erc-speedbar and
erc-status-sidebar.
(erc-open): Run `erc--setup-buffer-hook' after `erc-setup-buffer' so
hook members know their code isn't tied to `erc-setup-buffer' itself,
which may be used in other contexts, but rather to a new ERC buffer on
which some display-related action has just been performed.
(erc--called-as-input-p): New variable for "slash" commands, like
`erc-cmd-FOO', to detect whether they're being called "interactively"
as a result of input given at ERC's prompt.
(erc-process-input-line): Bind `erc--called-as-input-p' when running
slash commands.
(erc-cmd-JOIN): When called interactively, schedule a callback to wrap
the response handler and control how new buffers are thus displayed.
(erc-cmd-QUERY): Use preferred alias for `erc-query-display'.
* test/lisp/erc/erc-scenarios-base-buffer-display.el:
(erc-scenarios-base-buffer-display--interactive-default): New test.
* test/lisp/erc/erc-tests.el (erc-process-input-line,
erc-select-read-args, erc-tls, erc--interactive): Change expected
default value of `erc-interactive-display' from `buffer' to
`window'.  (Bug#62833)