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/unlink.cpp

@@ -19,7 +19,7 @@ ErrorOr<FlatPtr> Process::sys$unlink(int dirfd, Userspace<char const*> user_path
     if (flags & ~AT_REMOVEDIR)
         return Error::from_errno(EINVAL);
 
-    RefPtr<Custody> base;
+    LockRefPtr<Custody> base;
     if (dirfd == AT_FDCWD) {
         base = current_directory();
     } else {