Kernel: Ensure only user processes are terminated properly in shutdown

This patch ensures that the shutdown procedure can complete due to the
fact we don't kill kernel processes anymore, and only stop the scheduler
from running after the filesystems unmount procedure.

We also need kernel processes during the shutdown procedure, because we
rely on the WorkQueue threads to run WorkQueue items to complete async
IO requests initiated by filesystem sync & unmounting, etc.

This is also simplifying the code around the killing processes, because
we don't need to worry about edge cases such as the FinalizerTask
anymore.

Kernel/Tasks/PowerStateSwitchTask.h

@@ -32,11 +32,7 @@ private:
     static ErrorOr<void> perform_reboot();
     static ErrorOr<void> perform_shutdown();
 
-    enum class ProcessKind {
-        User,
-        Kernel,
-    };
-    static ErrorOr<void> kill_processes(ProcessKind, ProcessID finalizer_pid);
+    static ErrorOr<void> kill_all_user_processes();
 };
 
 }