Add separate bitcoin-rpc client

This adds an executable `bitcoin-rpc` that only serves as a Bitcoin RPC
client.
The commit does not remove RPC functionality from the `bitcoind` yet,
this functionality should be deprecated but is left for a later version
to give users some time to switch.

src/init.h

@@ -14,6 +14,15 @@ void StartShutdown();
 bool ShutdownRequested();
 void Shutdown();
 bool AppInit2(boost::thread_group& threadGroup);
-std::string HelpMessage();
+
+/* The help message mode determines what help message to show */
+enum HelpMessageMode
+{
+    HMM_BITCOIND,
+    HMM_BITCOIN_QT,
+    HMM_BITCOIN_CLI
+};
+
+std::string HelpMessage(HelpMessageMode mode);
 
 #endif