add chess960 support

[robertnurnberg]

The title of the PR is of course tongue in cheek. Discussing this with noob on discord we realized that the functionality is already there, we just need to inform the user how to encode castling rights in the fen (like we do for ep squares now).

Indeed, with the books from official-stockfish/books#71 I get the following results:

> ./cdbdirect_threaded DFRC_openings.epd 
Loading: DFRC_openings.epd
Opened DB with 53035759834 stored positions.
Probing 921600 fens with 24 threads.
known fens:         921600  ( 100.00% )
unknown fens:            0  (  0.00% )
scored moves:      4839190  ( 5.25 per known fen )
Required probing time: 3.56 sec.
Required time per fen: 3.87 microsec.
Known evals written to cdbdirect.epd.
Closing DB
> ./cdbdirect_threaded DFRC_4852_v1.epd 
Loading: DFRC_4852_v1.epd
Opened DB with 53035759834 stored positions.
Probing 294466 fens with 24 threads.
known fens:         294466  ( 100.00% )
unknown fens:            0  (  0.00% )
scored moves:      1552187  ( 5.27 per known fen )
Required probing time: 10.03 sec.
Required time per fen: 34.07 microsec.
Known evals written to cdbdirect.epd.
Closing DB