libroot: spawn_thread() now creates a detached pthread.

* __pthread_destroy_thread() will in turn free the pthread_thread object.
* this fixes a leak of 2072 bytes on each thread construction/destruction
and #9945. MediaExtractor spawns a thread on construction, which leaked
its pthread_thread object on destuction.

src/system/libroot/os/thread.c

@@ -110,6 +110,7 @@ spawn_thread(thread_func entry, const char *name, int32 priority, void *data)
 
 	__pthread_init_creation_attributes(NULL, thread, &thread_entry, entry,
 		thread, name, &attributes);
+	thread->flags |= THREAD_DETACHED;
 
 	attributes.priority = priority;
 
@@ -256,4 +257,3 @@ snooze_until(bigtime_t timeout, int timeBase)
 {
 	return _kern_snooze_etc(timeout, timeBase, B_ABSOLUTE_TIMEOUT, NULL);
 }
-