country.sys: codepage 667/790/991 (Mazovia) support

[mateuszviste]

The EDR country.sys supports Poland NLS under codepage 852. This mimics what MS-DOS did back in the day, but it wasn't a popular choice in Poland since Poles did not wait for Microsoft and created their own codepage named Mazovia that was far superior to the CP852 proposition.

FreeDOS (and SvarDOS) uses Mazovia for LANG=PL, so perhaps it would be possible to port this from their country.sys into the EDR country.sys? Alternatively, #51 would make this subject a non-issue.