Improvements towards conformity with XT standard

[izne]

Hello Gentlemen,

As I was preparing to fork this nice project to work on adding a new feature, I noticed an existing fork that adds the following improvements:

• IO changed to "open collector" to minimize the risk on shorting pins by mistake(frying the XT) and to conform with standard
• Fixes issue that would cause an IBM XT to enter "Manufacturing mode" after POST so keyboard would only work 50% of the time in Cassette Basic

I just want to highlight about the possibility for these improvements to be pulled into the original repo.

Thank you for your work and contributions!