bug#50043: 28.0.50; USABLE_SIGOI undef code paths do not work correctly

[Eli Zaretskii]

> Date: Wed, 17 Nov 2021 12:25:38 -0500
> Cc: larsi@gnus.org, 50043@debbugs.gnu.org
> From: Ken Brown <kbrown@cornell.edu>
> 
> There's still one more issue related to this bug, which I mentioned earlier: 
> If 
> x_selection_timeout is zero, x_get_foreign_selection will call 
> wait_reading_process output with an infinite timeout rather than a zero 
> timeout, 
> which I'm sure is not what was intended.  The attached patch fixes that.

Bu that's clearly what was intended.  The doc string of
x-selection-timeout says:

  A value of 0 means wait as long as necessary.

And with the fix in wait_reading_process_output the original problem
is gone, since we will now loop with short select timeout, waiting for
the selection event.  Which is the intent here, AFAIU.