bug#15180: Add a way to fflush stdout in batch mode

[Lars Ingebrigtsen <larsi@gnus.org>]

Stefan Monnier <monnier@IRO.UMontreal.CA> writes:

>>> What is the function to flush a stdio stream from Elisp? If it exists then
>>> it could be called before the sleep-for.
>>> An alternative to adding such a function is to simply have the print
>>> functions call fflush in C if noninteractive.
>> That function would be awesome. Is it possible to call such a C function
>> from Elisp?
>
> I guess it would make sense to call fflush in the batch equivalent
> of redisplay.

That sounds attractive, but I think there's a real possibility that that
could have unforeseen effects in older scripts -- uncomplete lines would
be output automatically, and that might break stuff.

So I've instead added a `flush-standard-output' function to Emacs 29, so
that the script writers can control this themselves.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no