Fixes for PR #762 (Musketeer variant)

[cross-of-north]

Fixed typos and other reasons why tests are failing in #762 . Now all tests should be green. Hope I haven't broke the original idea.

[replacement of #771]

[musketeerchess]

Hi, i tested the executables from these changes and they perfectly work.

Fabian, can you check them yourself and then merge with main code? Thanks

[cross-of-north]

After the initial iteration was done I've added evaluation-related changes (based on https://github.com/ianfab/Musketeer-Stockfish) which allow engine not just to play musketeer variant but to win: https://github.com/cross-of-north/Fairy-Stockfish/compare/bf7b3343f44cf7210c88d58e58fe0067af347c7b...9d81d08fe655c7de49a06cb6f80b78c5e2b31d6a

@ianfab, please suggest: should I add evaluation changes in this PR or create another? Musketeer-related evaluation changes add descriptions for musketeer-specific pieces into common engine parts; I don't know if it is right (how much engine should remain neutral, keeping specifics isolated in variants).

[ianfab]

@cross-of-north Thanks. Please keep playing strength improvements and game logic in separate PRs, because the former is more optional and therefore more likely to be rejected if changes are too big and gains too specific to a single variant.

Also please note that changes removing existing features, e.g., replacing custom piece slots by built-in types, have very few chance of getting merged, since the benefits would need to significantly outweigh the downsides of the removed flexibility.

[cross-of-north]

I suppose that there will be no more changes in this branch. So it's ready for review and/or merge.