add C11 condition variable functions

Because of the clear separation for private pthread_cond_t these
interfaces are quite simple and direct.

src/thread/cnd_broadcast.c

+#include <threads.h>
+
+int __private_cond_signal(cnd_t *, int);
+
+int cnd_broadcast(cnd_t *c)
+{
+	/* This internal function never fails, and always returns zero,
+	 * which matches the value thrd_success is defined with. */
+	return __private_cond_signal(c, -1);
+}

src/thread/cnd_destroy.c

+#include <threads.h>
+
+void cnd_destroy(cnd_t *c)
+{
+	/* For private cv this is a no-op */
+}

src/thread/cnd_init.c

+#include <threads.h>
+
+int cnd_init(cnd_t *c)
+{
+	*c = (cnd_t){ 0 };
+	return thrd_success;
+}

src/thread/cnd_signal.c

+#include <threads.h>
+
+int __private_cond_signal(cnd_t *, int);
+
+int cnd_signal(cnd_t *c)
+{
+	/* This internal function never fails, and always returns zero,
+	 * which matches the value thrd_success is defined with. */
+	return __private_cond_signal(c, 1);
+}

src/thread/cnd_timedwait.c

+#include <threads.h>
+#include <errno.h>
+
+int __pthread_cond_timedwait(cnd_t *restrict, mtx_t *restrict, const struct timespec *restrict);
+
+int cnd_timedwait(cnd_t *restrict c, mtx_t *restrict m, const struct timespec *restrict ts)
+{
+	int ret = __pthread_cond_timedwait(c, m, ts);
+	switch (ret) {
+	/* May also return EINVAL or EPERM. */
+	default:        return thrd_error;
+	case 0:         return thrd_success;
+	case ETIMEDOUT: return thrd_timedout;
+	}
+}

src/thread/cnd_wait.c

+#include <threads.h>
+
+int cnd_wait(cnd_t *c, mtx_t *m)
+{
+	/* Calling cnd_timedwait with a null pointer is an extension.
+	 * It is convenient here to avoid duplication of the logic
+	 * for return values. */
+	return cnd_timedwait(c, m, 0);
+}