qt: allow walletpassphrase in debug console without -server

Currently it is only possible to use `walletpassphrase` to unlock the
wallet when bitcoin is started in server mode.

Almost everything that manipulates the wallet in the RPC console
needs the wallet to be unlocked and is thus unusable without -server.

This is pretty unintuitive to me, and I'm sure it's even more confusing
to users.

Solve this with a very minimal change: by making the GUI start a
dummy RPC thread just to handle timeouts.

src/rpcserver.h

@@ -20,7 +20,14 @@
 
 class CBlockIndex;
 
+/* Start RPC threads */
 void StartRPCThreads();
+/* Alternative to StartRPCThreads for the GUI, when no server is
+ * used. The RPC thread in this case is only used to handle timeouts.
+ * If real RPC threads have already been started this is a no-op.
+ */
+void StartDummyRPCThread();
+/* Stop RPC threads */
 void StopRPCThreads();
 
 /*