bug#51820: 29.0.50; process_pending_signals is almost useless without SIGIO

[Ken Brown <kbrown@cornell.edu>]

I might be missing something, but it seems to me that the main purpose of 
process_pending_signals is to read input in case the user typed C-g.  This is in 
fact done on systems with SIGIO, via a call to handle_async_input.  But 
handle_async_input is a NOOP on systems without SIGIO.  This has been the case 
since the following commit:

commit 4d7e6e51dd4acecff466a28d958c50f34fc130b8
Author: Paul Eggert <eggert@cs.ucla.edu>
Date:   Sun Sep 23 01:44:20 2012 -0700

     Simplify and avoid signal-handling races.
[...]
     Fixes: debbugs:12471

I can't find anything in the commit message or in the discussion of Bug#12471 
that explains this.  Paul, I realize that the commit was 9 years ago, but do you 
remember why you made this change?

On systems without SIGIO, an atimer "poll_timer" is created that fires every 2 
seconds.  This causes pending_signals to be set, which has practically no effect 
AFAICT.  The only thing I see that poll_timer accomplishes is that if emacs 
happens to be in the select call in wait_reading_process_output when the timer 
fires, then select will return.

Ken