refactor: Add InitContext function to initialize NodeContext with global pointers

Having InitContext() avoids the need to add duplicate code to src/init/*.cpp
files in the next commit. It also lets these files avoid referencing global
variables like gArgs.

There is no change in behavior in this commit.

src/init.h

@@ -26,6 +26,9 @@ namespace node {
 struct NodeContext;
 } // namespace node
 
+/** Initialize node context variables. */
+void InitContext(node::NodeContext& node);
+
 /** Interrupt threads */
 void Interrupt(node::NodeContext& node);
 void Shutdown(node::NodeContext& node);