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/TTY/PTYMultiplexer.cpp

@@ -38,9 +38,9 @@ UNMAP_AFTER_INIT void PTYMultiplexer::initialize()
     the().after_inserting();
 }
 
-ErrorOr<NonnullRefPtr<OpenFileDescription>> PTYMultiplexer::open(int options)
+ErrorOr<NonnullLockRefPtr<OpenFileDescription>> PTYMultiplexer::open(int options)
 {
-    return m_freelist.with([&](auto& freelist) -> ErrorOr<NonnullRefPtr<OpenFileDescription>> {
+    return m_freelist.with([&](auto& freelist) -> ErrorOr<NonnullLockRefPtr<OpenFileDescription>> {
         if (freelist.is_empty())
             return EBUSY;