Kernel: Introduce the DeviceManagement singleton

This singleton simplifies many aspects that we struggled with before:
1. There's no need to make derived classes of Device expose the
constructor as public anymore. The singleton is a friend of them, so he
can call the constructor. This solves the issue with try_create_device
helper neatly, hopefully for good.
2. Getting a reference of the NullDevice is now being done from this
singleton, which means that NullDevice no longer needs to use its own
singleton, and we can apply the try_create_device helper on it too :)
3. We can now defer registration completely after the Device constructor
which means the Device constructor is merely assigning the major and
minor numbers of the Device, and the try_create_device helper ensures it
calls the after_inserting method immediately after construction. This
creates a great opportunity to make registration more OOM-safe.

Kernel/Devices/ZeroDevice.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/Memory.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/ZeroDevice.h>
 #include <Kernel/Sections.h>
 
@@ -12,7 +13,7 @@ namespace Kernel {
 
 UNMAP_AFTER_INIT NonnullRefPtr<ZeroDevice> ZeroDevice::must_create()
 {
-    auto zero_device_or_error = try_create_device<ZeroDevice>();
+    auto zero_device_or_error = DeviceManagement::try_create_device<ZeroDevice>();
     // FIXME: Find a way to propagate errors
     VERIFY(!zero_device_or_error.is_error());
     return zero_device_or_error.release_value();