bug#31039: 27.0.50; allow silencing auto-save

[Aaron Jensen]

Currently, keyboard.c calls do-auto-save directly and passes nil to the
NO-MESSAGE arg. It would be nice if there was a way to pass t in these
situations in order to suppress the message. Perhaps a new var to
suppress the auto-save message in all cases?

This has been asked about and/or hacked around (by disabling the normal
auto-save mechanisms and adding an idle timer) a number of times:

https://www.reddit.com/r/emacs/comments/63gmhd/silencing_autosave_messages/
https://emacs.stackexchange.com/questions/12556/disabling-the-auto-saving-done-message
http://pragmaticemacs.com/emacs/make-emacs-a-bit-quieter/