multiprocess: Add bitcoin-node process spawning support

Add bitcoin-node startup code to let it spawn and be spawned by other
processes

src/interfaces/init.h

@@ -31,6 +31,20 @@ public:
     virtual std::unique_ptr<WalletClient> makeWalletClient(Chain& chain);
     virtual Ipc* ipc();
 };
+
+//! Return implementation of Init interface for the node process. If the argv
+//! indicates that this is a child process spawned to handle requests from a
+//! parent process, this blocks and handles requests, then returns null and a
+//! status code to exit with. If this returns non-null, the caller can start up
+//! normally and use the Init object to spawn and connect to other processes
+//! while it is running.
+std::unique_ptr<Init> MakeNodeInit(NodeContext& node, int argc, char* argv[], int& exit_status);
+
+//! Return implementation of Init interface for the wallet process.
+std::unique_ptr<Init> MakeWalletInit(int argc, char* argv[], int& exit_status);
+
+//! Return implementation of Init interface for the gui process.
+std::unique_ptr<Init> MakeGuiInit(int argc, char* argv[]);
 } // namespace interfaces
 
 #endif // BITCOIN_INTERFACES_INIT_H