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/Syscalls/fork.cpp

@@ -17,7 +17,7 @@ ErrorOr<FlatPtr> Process::sys$fork(RegisterState& regs)
 {
     VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this);
     TRY(require_promise(Pledge::proc));
-    RefPtr<Thread> child_first_thread;
+    LockRefPtr<Thread> child_first_thread;
 
     ArmedScopeGuard thread_finalizer_guard = [&child_first_thread]() {
         SpinlockLocker lock(g_scheduler_lock);