Kernel+Userland: Introduce a new way to reboot and poweroff the machine

This change removes the halt and reboot syscalls, and create a new
mechanism to change the power state of the machine.
Instead of how power state was changed until now, put a SysFS node as
writable only for the superuser, that with a defined value, can result
in either reboot or poweroff.
In the future, a power group can be assigned to this node (which will be
the GroupID responsible for power management).

This opens an opportunity to permit to shutdown/reboot without superuser
permissions, so in the future, a userspace daemon can take control of
this node to perform power management operations without superuser
permissions, if we enforce different UserID/GroupID on that node.

Kernel/Firmware/PowerStateSwitch.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefPtr.h>
+#include <AK/Types.h>
+#include <AK/Vector.h>
+#include <Kernel/FileSystem/SysFS.h>
+#include <Kernel/Firmware/SysFSFirmware.h>
+#include <Kernel/KBuffer.h>
+#include <Kernel/Memory/MappedROM.h>
+#include <Kernel/Memory/Region.h>
+#include <Kernel/PhysicalAddress.h>
+#include <Kernel/VirtualAddress.h>
+
+namespace Kernel {
+
+class PowerStateSwitchNode final : public SysFSComponent {
+public:
+    static NonnullRefPtr<PowerStateSwitchNode> must_create(FirmwareSysFSDirectory&);
+    virtual mode_t permissions() const override;
+    virtual KResultOr<size_t> write_bytes(off_t, size_t, UserOrKernelBuffer const&, OpenFileDescription*) override;
+
+private:
+    PowerStateSwitchNode(FirmwareSysFSDirectory&);
+
+    void reboot();
+    void poweroff();
+};
+
+}