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/SyncEvent.h

@@ -29,6 +29,7 @@ namespace VeraCrypt
 		SyncEvent ();
 		~SyncEvent ();
 
+		void Reset ();
 		void Signal ();
 		void Wait ();