Remove signal DoubleSpendDetected, use function

Also removes the need for forward reference to RelayableRespend.

src/init.cpp

@@ -1176,7 +1176,7 @@ bool AppInit2(boost::thread_group& threadGroup)
     LogPrintf("mapAddressBook.size() = %u\n",  pwalletMain ? pwalletMain->mapAddressBook.size() : 0);
 #endif
 
-    RegisterInternalSignals();
+    InitRespendFilter();
     StartNode(threadGroup);
     if (fServer)
         StartRPCThreads();