gh-59705: Add _thread.set_name() function

[vstinner]

On Linux, threading.Thread now sets the thread name to the operating system.

configure now checks if pthread_setname_np() function is available.

• Issue: Python should support exporting thread names to the OS #59705

[vstinner]

This implementation is very basic on purpose. I plan to add support for more platform in follow-up PRs.

• On Linux, set_name() does nothing if the name is longer than 15 bytes. Should the function truncate silently to 15 bytes instead? I don't think that raising an exception is very convenient here.
• Setting Thread.name after Thread.start() doesn't call again set_name(). set_name() is called only once per thread, at startup.
• I didn't add automated tests since I don't want to add a get_name() function (use Thread.name to get a thread name).

Demo 1 (main thread):

$ ./python
>>> import os
>>> pid=os.getpid()
>>> with open(f"/proc/{pid}/task/{pid}/comm") as fp: print(f"comm = {fp.read()!r}")
... 
comm = 'python\n'

>>> import _thread; _thread.set_name("demo")
>>> with open(f"/proc/{pid}/task/{pid}/comm") as fp: print(f"comm = {fp.read()!r}")
... 
comm = 'demo\n'

Demo 2 (thread):

$ ./python
>>> import threading, os, time
>>> os.getpid()
81921
>>> t=threading.Thread(target=time.sleep, args=(60,), name="sleeper")
>>> t.start()
^Z

$ cat /proc/81921/task/81927/comm 
sleeper

See also a previous attempt to implement the feature: #14578

[vstinner]

I didn't add automated tests since I don't want to add a get_name() function (use Thread.name to get a thread name).

I changed my mind and added a private _thread._get_name() function for tests.