Kernel: Move TTY-related code to a new subdirectory under Devices

The TTY subsystem is represented with unix devices, so it should be
under the Devices directory like the Audio, Storage, GPU and HID
subsystems.

Kernel/Devices/TTY/PTYMultiplexer.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Badge.h>
+#include <Kernel/Devices/CharacterDevice.h>
+
+namespace Kernel {
+
+class MasterPTY;
+
+class PTYMultiplexer final : public CharacterDevice {
+public:
+    PTYMultiplexer();
+    virtual ~PTYMultiplexer() override;
+
+    static void initialize();
+    static PTYMultiplexer& the();
+
+    // ^CharacterDevice
+    virtual ErrorOr<NonnullRefPtr<OpenFileDescription>> open(int options) override;
+    virtual ErrorOr<size_t> read(OpenFileDescription&, u64, UserOrKernelBuffer&, size_t) override { return 0; }
+    virtual ErrorOr<size_t> write(OpenFileDescription&, u64, UserOrKernelBuffer const&, size_t) override { return 0; }
+    virtual bool can_read(OpenFileDescription const&, u64) const override { return true; }
+    virtual bool can_write(OpenFileDescription const&, u64) const override { return true; }
+
+    void notify_master_destroyed(Badge<MasterPTY>, unsigned index);
+
+private:
+    // ^CharacterDevice
+    virtual StringView class_name() const override { return "PTYMultiplexer"sv; }
+
+    static constexpr size_t max_pty_pairs = 64;
+    SpinlockProtected<Vector<unsigned, max_pty_pairs>, LockRank::None> m_freelist {};
+};
+
+}