GUS support

Version 33 introduced user interface for Gravis UltraSound. This UI was added because some games won't start with Sound Blaster, but with GUS works well. GUS requires special external files what must be placed into diretory ULTRASND411 (by default). And this directory must be placed in directory what is mounted to Drive C:\
Example
Path /storage/emulated/0/Games is mounted to Drive C:\. Then the directory ULTRASND411 must be placed here /storage/emulated/0/Games/ULTRASND411.
Warning
These data can't be part of Magic Dosbox application.