[NTOSKRNL] Rewrite IoAssignDriveLetters to make NT5 compliant

[NTOSKRNL] Rewrite IoAssignDriveLetters to make NT5 compliant

The major change with this rewrite is the support for the mount
manager. Fstub will now assume that most of the devices are PnP
and that they are already registered to the mount manager.
It will thus ask the mount manager to assign the drive letter.
Fstub will keep assigning drive letters non mission critical devices
such as CDs, floppies and other removable devices.

See MountMgr:QueryPoints API test that will now return mount points :-).