Track book index per game

[ripred]

Summary

• add book_index to the game state
• use book_index in check_book
• reset book_index in game_t::init
• add Arduino stub headers so code compiles in a non-Arduino environment

Testing

• g++ -I. -c board.cpp
• g++ -I. -c test/unit_test_001.cpp
• g++ -I. -c game.cpp
• g++ -I. -c chessutil.cpp (warns: second parameter of 'va_start' not last named argument)