bug#44018: Don't consider play-sound-file to be a 'safe' function

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Mon, 26 Oct 2020 21:36:50 +0100
> Cc: contovob@tcd.ie, larsi@gnus.org, 44018@debbugs.gnu.org
> 
> 26 okt. 2020 kl. 19.28 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> > So you are saying 'message' is not safe because it could be used to
> > display text that isn't "innocuous"?  In that case, we should also
> > remove from the list 'error' and 'signal', no?
> 
> That's a good point. Perhaps we should.
> There doesn't seem to be much point in permitting 'while' either in the 
> absence of mutation, but I don't think it's actively harmful. I could be 
> wrong.

I'm okay with doing this, but please add commentary that explains
these removals and the general policy for considering a function
"safe".

Thanks.