codestyle cleanup in board
* Use tabs instead of spaces. This is important, because we all use different text editors and diffent nb of spaces per tab. We can choose to display tabs as 2,4,8 spaces in our editors, but the encoding in the file must always use tabs to keep the flexibility.
* Rename en_passant_square to epsq in struct Board. Short and sweet and just as self-documenting.
* When there are several int parameters to a function or an array, to be consistent across the code, always sort by ascending cardinality. So it should be holdings[color][piece] not holdings[piece][color].
* Display the FEN in print_board(). Removes a bunch of ugly code, and the FEN contains -- or will contain -- all the info anyway (holdings, side to move, en passant square, etc.)
* Introduce helper function has_holdings().

No functional change.