bug#12471: Avoid some signal-handling races, and simplify.

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Paul Eggert <eggert@cs.ucla.edu>,  12471@debbugs.gnu.org,  
> lekktu@gmail.com
> Date: Sat, 22 Sep 2012 16:28:26 -0400
> 
> > But for existing library functions, the only sane way to replace them
> > is to have the replacement support all the features supported by the
> > function being replaced.
> 
> Agreed.  Maybe a better solution is to use `emacs_raise' which can then
> either use `raise' (on POSIX hosts) or something else (on Windows
> hosts).

Yes, that'd be good, I think.