Linux: parallelize header KDF autodetection

Extend the Unix encryption thread pool to run key-derivation work items and use it when mounting volumes without an explicitly selected KDF. This brings Linux/macOS header PRF autodetection closer to the Windows path while keeping selected-KDF mounts unchanged.

Fixes #1610.

src/Platform/Unix/SyncEvent.cpp

@@ -41,6 +41,14 @@ namespace VeraCrypt
 		Initialized = false;
 	}
 
+	void SyncEvent::Reset ()
+	{
+		assert (Initialized);
+
+		ScopeLock lock (EventMutex);
+		Signaled = false;
+	}
+
 	void SyncEvent::Signal ()
 	{
 		assert (Initialized);