AK: Add a ScopeGuard helper that invokes a callback when destroyed.

This is useful when you want to ensure some little thing happens when you
exit a certain scope.

This patch makes use of it in LibC's netdb code to make sure we close the
connection to the LookupServer.

AK/ScopeGuard.h

@@ -0,0 +1,24 @@
+#pragma once
+
+namespace AK {
+
+template<typename Callback>
+class ScopeGuard {
+public:
+    ScopeGuard(Callback callback)
+        : m_callback(move(callback))
+    {
+    }
+
+    ~ScopeGuard()
+    {
+        m_callback();
+    }
+
+private:
+    Callback m_callback;
+};
+
+}
+
+using AK::ScopeGuard;