[FASTFAT] Introduce a KDBG extension.

This is a PoC of what it's possible to realize thanks to an
already existing hack in ntoskrnl :-).
With this extension, on the kdb:> prompt, you're able to type
in commands that will be handled by the FastFAT driver and not
by the kernel, allowing internal debug, not possible otherwise.

So far, three commands exist:
- ?fat.vols: lists all the mounted volumes by FastFAT
- ?fat.files: lists all the files on a specific volume (with their attributes)
- ?fat.setdbgfile: allows watching on specifics files lifetime

This is obviously only the begin and could be greatly improved.

For instance, this is what allowed to debug CORE-14557

drivers/filesystems/fastfat/close.c

@@ -48,6 +48,9 @@ VfatCloseFile(
     }
     else
     {
+#ifdef KDBG
+        pFcb->Flags |= FCB_CLOSED;
+#endif
         vfatReleaseFCB(DeviceExt, pFcb);
     }