gh-154283: Make threading.get_native_id() unique across processes on DragonFly (GH-154287)

On DragonFly BSD lwp_gettid() is only unique within a process (the main
thread is always LWP 1), so combine it with the process id, like Solaris.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Misc/NEWS.d/next/Library/2026-07-20-21-36-40.gh-issue-154283.Xip7xE.rst

@@ -0,0 +1,2 @@
+On DragonFly BSD, :func:`threading.get_native_id` now returns a value that is
+unique across processes, matching the other platforms.

Python/thread_pthread.h

@@ -399,8 +399,8 @@ PyThread_get_thread_native_id(void)
     lwpid_t native_id;
     native_id = _lwp_self();
 #elif defined(__DragonFly__)
-    lwpid_t native_id;
-    native_id = lwp_gettid();
+    // lwp_gettid() is only unique within a process, so combine it with the pid.
+    unsigned long native_id = (unsigned long)getpid() << 32 | lwp_gettid();
 #elif defined(__sun__) && SIZEOF_LONG >= 8
     unsigned long native_id = (unsigned long)getpid() << 32 | thr_self();
 #endif