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

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: yantar92@gmail.com,  38632@debbugs.gnu.org
> Date: Wed, 18 Dec 2019 18:04:53 +0100
> 
>     Eli> (Btw, the limitation is 16 bytes, including the terminating null, so
>     Eli> truncation needs to be clever about non-ASCII characters, and I 
> wonder
>     Eli> what does prctl do when 15 bytes end in the middle of a multibyte
>     Eli> sequence.)
> 
> It does exactly what you'd expect, it drops the extraneous bytes, so
> putting eg ü on the boundary results in a name ending in à (#xc3).

So it is one more reason to do or own truncation, so we do it right.

> In any case, if emacs or prctl truncates, then the name as reported
> by 'list-threads' will be out of sync with pthread_getname_np, unless
> you'd want to adjust that too.

We need to truncate the names we store in the thread object (and
document that).