A synchronization semaphore. A synchronization semaphore which can be used to synchronize multiple [Thread]s. Initialized to zero on creation. Be careful to avoid deadlocks. For a binary version, see [Mutex]. $DOCS_URL/tutorials/threads/using_multiple_threads.html Lowers the [Semaphore], allowing one more thread in. Returns [constant OK] on success, [constant ERR_BUSY] otherwise. Tries to wait for the [Semaphore], if its value is zero, blocks until non-zero. Returns [constant OK] on success, [constant ERR_BUSY] otherwise.