AK: Add Eternal<T> and use it in various places.

This is useful for static locals that never need to be destroyed:

Thing& Thing::the()
{
    static Eternal<Thing> the;
    return the;
}

The object will be allocated in data segment memory and will never have
its destructor invoked.

Kernel/Net/LoopbackAdapter.cpp

@@ -1,11 +1,10 @@
 #include <Kernel/Net/LoopbackAdapter.h>
+#include <AK/Eternal.h>
 
 LoopbackAdapter& LoopbackAdapter::the()
 {
-    static LoopbackAdapter* the;
-    if (!the)
-        the = new LoopbackAdapter;
-    return *the;
+    static Eternal<LoopbackAdapter> the;
+    return the;
 }
 
 LoopbackAdapter::LoopbackAdapter()