bug#31558: 27.0; `custom-file' settings messed up by Emacs 27

[Drew Adams]

> I've checked all the defcustoms in Tramp. Looks, like only some few need
> the :require attribute in their spec, likely I have added this attribute
> too eagerly to *all* defcustoms ('cos "it doesn't hurt"). I have removed
> it from the majority of defcustoms, lets see how it works.
> 
> Pushed to master.

Thanks for doing this, Michael.