bug#38632: 27.0.50; Emacs process name is changed permanently upon creating a named thread

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Tue, 7 Jan 2020 17:46:49 +0100
> Cc: 38632@debbugs.gnu.org, rpluim@gmail.com
> 
> However, isn't thread_start_address inherently racy? It doesn't matter much 
> since it will only ever have a single value (run_thread), but it does violate 
> layering. That problem won't be addressed in this patch, however.

Yes, that should be a separate cleanup (and yes, we always set it to
the same function, so the race shouldn't matter in practice).