Get rid of shutdown.cpp/shutdown.h, use SignalInterrupt directly

This change is mostly a refectoring that removes some code and gets rid of an
unnecessary layer of indirection after #27861

But it is not a pure refactoring since StartShutdown, AbortShutdown, and
WaitForShutdown functions used to abort on failure, and the replacement code
logs or returns errors instead.

src/init.h

@@ -26,8 +26,10 @@ namespace node {
 struct NodeContext;
 } // namespace node
 
-/** Initialize node context variables. */
+/** Initialize node context shutdown and args variables. */
 void InitContext(node::NodeContext& node);
+/** Return whether node shutdown was requested. */
+bool ShutdownRequested(node::NodeContext& node);
 
 /** Interrupt threads */
 void Interrupt(node::NodeContext& node);