lib: Add semaphores

include/zinnia/semaphore.h

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: ISC
+
+#pragma once
+
+#include <stdint.h>
+
+typedef struct {
+    _Atomic(uint32_t) count;
+} sem_t;
+
+void sem_init(sem_t* sem, uint32_t count);
+bool sem_trywait(sem_t* sem);
+bool sem_timedwait(sem_t* sem, const uint64_t* deadline_ns);
+void sem_wait(sem_t* sem);
+void sem_post(sem_t* sem);