With visible-completions, only bind RET when completion is selected

With visible-completions, only bind RET when completion is selected

Previously, if minibuffer-visible-completions was non-nil, we bound RET
whenever the *Completions* buffer was visible.  This meant that RET in
completion-in-region would not enter a newline, which is a somewhat
annoying behavior change from minibuffer-visible-completions=nil.

Now, we only bind RET when a completion is selected.  This means
RET will newline in completion-in-region.

So that completion help continues to suggest the correct keys,
we also add minibuffer-visible-completions--always-bind.  When
let-bound to a non-nil value, it makes the
minibuffer-visible-completions binds always active.  We let-bind
it around substitute-command-keys.

* lisp/minibuffer.el (minibuffer-visible-completions--always-bind)
(minibuffer-visible-completions--filter): Add.
(minibuffer-visible-completions-bind): Use
minibuffer-visible-completions--filter.  (bug#68801)
* lisp/simple.el (minibuffer-visible-completions--always-bind)
(completion-setup-function): Let-bind
minibuffer-visible-completions--always-bind so the completion
help is correct.