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/bitcoin-gui.cpp

@@ -2,7 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include <common/args.h>
+#include <init.h>
 #include <interfaces/chain.h>
 #include <interfaces/echo.h>
 #include <interfaces/init.h>
@@ -23,7 +23,7 @@ class BitcoinGuiInit : public interfaces::Init
 public:
     BitcoinGuiInit(const char* arg0) : m_ipc(interfaces::MakeIpc(EXE_NAME, arg0, *this))
     {
-        m_node.args = &gArgs;
+        InitContext(m_node);
         m_node.init = this;
     }
     std::unique_ptr<interfaces::Node> makeNode() override { return interfaces::MakeNode(m_node); }