Kernel: Make self-contained locking smart pointers their own classes

Until now, our kernel has reimplemented a number of AK classes to
provide automatic internal locking:

- RefPtr
- NonnullRefPtr
- WeakPtr
- Weakable

This patch renames the Kernel classes so that they can coexist with
the original AK classes:

- RefPtr => LockRefPtr
- NonnullRefPtr => NonnullLockRefPtr
- WeakPtr => LockWeakPtr
- Weakable => LockWeakable

The goal here is to eventually get rid of the Lock* classes in favor of
using external locking.

Kernel/Locking/Mutex.h

@@ -100,7 +100,7 @@ private:
     // the lock is unlocked, it just means we don't know which threads hold it.
     // When locked exclusively, this is always the one thread that holds the
     // lock.
-    RefPtr<Thread> m_holder;
+    LockRefPtr<Thread> m_holder;
     size_t m_shared_holders { 0 };
 
     struct BlockedThreadLists {