Simplify multi-frame behavior in erc-bufbar-mode

Simplify multi-frame behavior in erc-bufbar-mode

* lisp/erc/erc-status-sidebar.el (erc-status-sidebar-singular,
erc-status-sidebar--singular-p): Replace option new in ERC 5.6 with
the latter, an internal flag.
(erc-status-sidebar-get-window): Use new name for option turned
ordinary variable `erc-status-sidebar--singular-p'.
(erc-status-sidebar-close): Add comment.
(erc-status-sidebar--open): New function containing the old body of
`erc-status-sidebar-open'.
(erc-bufbar-mode, erc-bufbar-enable, erc-bufbar-disable): Update
variable names.  Close sidebar window on all frames when disabling,
and don't set mode variable to nil when enabling.  These may have made
some practical sense but were illogical.  For example, it's confusing
to leave `erc-status-sidebar--open' in `erc--setup-buffer-hook' while
reporting the mode as being disabled.
(erc-status-sidebar-open): Move to slightly later in same file, after
defining `erc-bufbar-mode'.  When `erc-bufbar-mode' is active, always
create a sidebar if needed, even when another frame is already
displaying one.
(erc-status-toggle-sidebar): When `erc-bufbar-mode' is disabled,
revert to pre-5.6 behavior.  When the module is enabled, adopt new
behavior of ensuring the current frame shows a sidebar, even if
another frame already has one.
(erc-status-sidebar-refresh): Save and restore `window-start' in all
windows showing a sidebar buffer after refreshing.  Update option and
variable names.
(erc-status-sidebar-refresh-triggers): Add doc string, noting that the
variable is set locally when the option
`erc-status-sidebar-highlight-active-buffer' is non-nil.
(erc-status-sidebar--highlight-refresh-triggers): New variable
containing additional triggers enabled when the option
`erc-status-highlight-active-buffer' is non-nil.
(erc-status-sidebar--refresh-unless-input): New function to run
`erc-status-sidebar-refresh' unless input is pending or the selected
window's buffer is a minibuffer.
(erc-status-sidebar--post-refresh): Call `erc-status-sidebar-refresh'
wrapper `erc-status-sidebar--refresh-unless-input' instead.
(erc-status-sidebar-set-window-preserve-size): Update var name to
`erc-status-sidebar--singular-p'.
(erc-status-sidebar-mode): Run `erc-status-sidebar--post-refresh' on
`window-selection-change-functions' globally when highlighting active
buffers.  (bug#63595)