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

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@gmail.com>
> Date: Mon, 16 Dec 2019 14:42:38 +0800
> 
> When I try to create a named thread like
> 
> (make-thread (lambda () (+ 1 2)) "test-emacs-async")
> 
> the emacs process gets renamed to "test-emacs-async" even after the
> thread returns. Making another thread with different name renames emacs
> process again.

I think, instead of calling prctl in systhread.c, we should call
pthread_set_name_np, and the configure-time test for prctl should be
replaced with a test for pthread_set_name_np.

Patches welcome.