Make gArgs aware of the arguments

gArgs knows what the available arguments are and their help. Getting
the help message is moved to gArgs and HelpMessage() is removed

src/interfaces/node.cpp

@@ -83,7 +83,7 @@ class NodeImpl : public Node
             StopMapPort();
         }
     }
-    std::string helpMessage(HelpMessageMode mode) override { return HelpMessage(mode); }
+    void setupServerArgs() override { return SetupServerArgs(); }
     bool getProxy(Network net, proxyType& proxy_info) override { return GetProxy(net, proxy_info); }
     size_t getNodeCount(CConnman::NumConnections flags) override
     {