bug#74617: 30.0.92; ffap-menu always displays the *Completions* buffer

[Daniel Mendler via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

`ffap-menu' automatically displays the *Completions* buffer by calling
`minibuffer-completion-help'. If an alternative minibuffer completion
system like Icomplete or Vertico is used, the *Completions* buffer is
not needed since the candidates are already displayed in the minibuffer.

I propose to either detect these alternative completion systems (e.g.,
by checking the value of the completing-read-function and/or the mode
variables) or to provide a way to disable the call to
`minibuffer-completion-help'.

Since the same problem is present in tmm.el, maybe a generic solution
could be provided by minibuffer.el? Option 1: A function
`minibuffer-completion-help-if-needed' could call
`minibuffer-completion-help' only if no other completion system is
detected. Option 2: A new function
`completing-read-display-help-function' could be added which defaults to
`minibuffer-completion-help' and which could be set to nil/ignore by
alternative completion UIs like Vertico. This function could be used by
tmm/ffap. Option 3: A new variable `minibuffer-inhibit-completion-help'
could be added which is checked by `minibuffer-completion-help' and
which could be set to t by alternative completion UIs.

I am happy to provide a patch for any of these approaches.