trusted-content: Adjust the last patch based on preliminary feedback

trusted-content: Adjust the last patch based on preliminary feedback

* lisp/files.el (trusted-content): Rename from `trusted-files`.
Update all references.

* lisp/progmodes/elisp-mode.el (lisp-interaction-mode):
* lisp/ielm.el (inferior-emacs-lisp-mode):
* lisp/simple.el (read--expression): Set `trusted-content` since
these buffers contain code that the user presumably intends to run anyway.
(elisp--safe-macroexpand-all): Make the warning more discreet.

(cherry picked from commit 8b6c6cffd1f772301e89353de5e057835af18a30)