Make threads faster and more reliable

This change doubles the performance of thread spawning. That's thanks to
our new stack manager, which allows us to avoid zeroing stacks. It gives
us 15µs spawns rather than 30µs spawns on Linux. Also, pthread_exit() is
faster now, since it doesn't need to acquire the pthread GIL. On NetBSD,
that helps us avoid allocating too many semaphores. Even if that happens
we're now able to survive semaphores running out and even memory running
out, when allocating *NSYNC waiter objects. I found a lot more rare bugs
in the POSIX threads runtime that could cause things to crash, if you've
got dozens of threads all spawning and joining dozens of threads. I want
cosmo to be world class production worthy for 2025 so happy holidays all

libc/thread/pthread_decimate_np.c

@@ -16,22 +16,32 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/cosmo.h"
+#include "libc/intrin/stack.h"
 #include "libc/thread/posixthread.internal.h"
 #include "libc/thread/thread.h"
 
 /**
  * Garbage collects POSIX threads runtime.
  *
- * Let's say you want to run a memory leak detector. You can say:
+ * This function frees unreferenced zombie threads and empties cache
+ * memory associated with the Cosmopolitan POSIX threads runtime.
+ *
+ * Here's an example use case for this function. Let's say you want to
+ * create a malloc() memory leak detector. If your program was running
+ * threads earlier, then there might still be allocations lingering
+ * around, that'll give you false positives. To fix this, what you would
+ * do is call the following, right before running your leak detector:
  *
  *     while (!pthread_orphan_np())
  *       pthread_decimate_np();
  *
- * To wait until all threads have exited.
+ * Which will wait until all threads have exited and their memory freed.
  *
  * @return 0 on success, or errno on error
  */
 int pthread_decimate_np(void) {
-  _pthread_decimate(false);
+  _pthread_decimate();
+  cosmo_stack_clear();
   return 0;
 }