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.cpp

@@ -192,6 +192,10 @@ static void RemovePidFile(const ArgsManager& args)
     }
 }
 
+void InitContext(NodeContext& node)
+{
+    node.args = &gArgs;
+}
 
 //////////////////////////////////////////////////////////////////////////////
 //