Use *NSYNC for POSIX threads locking APIs

Condition variables, barriers, and r/w locks now work very well.

third_party/nsync/mu_semaphore.h

@@ -0,0 +1,28 @@
+#ifndef NSYNC_SEM_H_
+#define NSYNC_SEM_H_
+#include "third_party/nsync/time.h"
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+typedef struct nsync_semaphore_s_ {
+  void *sem_space[32]; /* space used by implementation */
+} nsync_semaphore;
+
+/* Initialize *s; the initial value is 0. */
+void nsync_mu_semaphore_init(nsync_semaphore *s);
+
+/* Wait until the count of *s exceeds 0, and decrement it. */
+void nsync_mu_semaphore_p(nsync_semaphore *s);
+
+/* Wait until one of: the count of *s is non-zero, in which case
+   decrement *s and return 0; or abs_deadline expires, in which case
+   return ETIMEDOUT. */
+int nsync_mu_semaphore_p_with_deadline(nsync_semaphore *s,
+                                       nsync_time abs_deadline);
+
+/* Ensure that the count of *s is at least 1. */
+void nsync_mu_semaphore_v(nsync_semaphore *s);
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* NSYNC_SEM_H_ */