* Import unmodified USE!UMBS source.
From https://forum.vcfed.org/index.php?threads/loading-dos-high-on-a-xt.32320/
* Customize the UMB Driver prompts.
* Refactor memory management. Allow activation of EMS/UMB on demand.
* Update the XTEMM and XTUMBS drivers to map their ranges.
* Add instructions for all the drivers.