Use *NSYNC for POSIX threads locking APIs

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

third_party/nsync/counter.h

@@ -0,0 +1,43 @@
+#ifndef NSYNC_COUNTER_H_
+#define NSYNC_COUNTER_H_
+#include "third_party/nsync/atomic.h"
+#include "third_party/nsync/time.h"
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+struct nsync_dll_element_s_;
+
+/* An nsync_counter represents an unsigned integer that can count up and down,
+   and wake waiters when zero.  */
+typedef struct nsync_counter_s_ *nsync_counter;
+
+/* Return a freshly allocated nsync_counter with the specified value,
+   of NULL if an nsync_counter cannot be created.
+
+   Any non-NULL returned value should be passed to nsync_counter_free() when no
+   longer needed.  */
+nsync_counter nsync_counter_new(uint32_t value);
+
+/* Free resources associated with c.  Requires that c was allocated by
+   nsync_counter_new(), and no concurrent or future operations are applied to
+   c.  */
+void nsync_counter_free(nsync_counter c);
+
+/* Add delta to c, and return its new value.  It is a checkable runtime error
+   to decrement c below 0, or to increment c (i.e., apply a delta > 0) after a
+   waiter has waited.  */
+uint32_t nsync_counter_add(nsync_counter c, int32_t delta);
+
+/* Return the current value of c.  */
+uint32_t nsync_counter_value(nsync_counter c);
+
+/* Wait until c has value 0, or until abs_deadline, then return
+   the value of c.  It is a checkable runtime error to increment c after
+   a waiter may have been woken due to the counter reaching zero.
+   If abs_deadline==nsync_time_no_deadline, the deadline
+   is far in the future. */
+uint32_t nsync_counter_wait(nsync_counter c, nsync_time abs_deadline);
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* NSYNC_COUNTER_H_ */