Get rid of kmalloc()

This changes *NSYNC to allocate waiters on the stack so our locks don't
need to depend on dynamic memory. This make our runtiem simpler, and it
also fixes bugs with thread cancellation support.

third_party/nsync/README.cosmo

@@ -15,9 +15,22 @@ ORIGIN
 
 LOCAL CHANGES
 
-  - nsync_malloc_() is implemented as kmalloc()
-  - nsync_mu_semaphore uses Cosmopolitan Futexes
-  - block pthread cancellations in nsync_mu_lock_slow_
-  - support posix thread cancellations in nsync_cv_wait
-  - timespec api was so good that it's now part of libc
-  - linked list api was so good that it's now part of libc
+  - Time APIs were so good that they're now part of our libc
+
+  - Double linked list API was so good that it's now part of our libc
+
+  - Modified *NSYNC to allocate waiter objects on the stack. We need it
+    because we use *NSYNC mutexes to implement POSIX mutexes, which are
+    too low-level to safely depend on malloc, or even mmap in our case.
+
+  - Rewrote most of the semaphore and futex system call support code so
+    it works well with Cosmopolitan's fat runtime portability. *NSYNC's
+    unit test suite passes on all supported platforms. However the BSDs
+    currently appear to be overutilizing CPU time compared with others.
+
+  - Support POSIX thread cancellation. APIs that wait on condition vars
+    are now cancellation points. In PTHREAD_CANCEL_MASKED mode they may
+    return ECANCELED. In PTHREAD_CANCEL_DEFERRED mode the POSIX threads
+    library will unwind the stack to unlock any locks and free waiters.
+    On the other hand the *NSYNC APIs for mutexes will now safely block
+    thread cancellation, but you can still use *NSYNC notes to do that.